Not So Different: Finding Human Nature in Animals

THE DISCUSSION OF jealousy begins a new and depressing unit in this book. Thus far, we have been talking about how kind and loving animals can be. We have talked about chimpanzees that took care of their developmentally disabled friend, a dog that grieved for a beaver, an elephant that refused to harm a dog, Canada geese that mate for life, dolphins that refuse to let go of their dead children, penguins that adopt abandoned children, wolves that insist on playing by the rules, and baboons that comforted a lonely mother who lost her child. We have been able to see the best of ourselves within these animals. This has been a heartwarming discussion of the virtues of fair, generous, and loving animals that parallel our own fairness, generosity, and love.

Now, we must talk about our dark side. Although we have endless capacity to do good and to love others selflessly, we also have a capacity for great evil. Ruthlessness can take many forms, but the unifying theme is total disregard for the well-being of others in order to single-mindedly further one’s own interests. Examples of this fill our history books and current events. The same goes for animal life in the wild. There is no shortage of ruthlessness in nature.

In the previous chapters, I have made a strong argument that the evolution of many animal lineages, including humans, is marked by cooperation, reciprocation, and social attachments. I made this argument to contrast the traditional view of natural selection as working solely through intense and ruthless competition. I have downplayed the competitive side of animal life because that has received enough attention. The truth is that both views are correct. The evolution of social animals has been a balancing act between cooperation and competition, and some species are more competitive while others are more cooperative.

For example, among our closest relatives, chimpanzees and bonobos are much more cooperative and social than are orangutans. While chimpanzee life can certainly involve aggression and fighting, it is also marked by peaceful dispute resolution, reciprocal altruism and generosity, and affection. Most chimpanzee fighting, in fact, is between-group hostility, not within-group. Orangutans, on the other hand, are solitary as adults and generally hostile to each other. The only real bonding that takes place is between mothers and their children, but even this occurs only when they are young and dependent. Mothers and their adult children show no special connection, affection, or cooperation. In fact, the impressive intelligence of the orangutan is a major challenge to the prevailing hypothesis that sociality is the key driving force for the evolution of higher cognitive abilities.1

Just as each species is different, individuals within a species can vary as well. Of course, we know that some humans are much more generous and compassionate than others, and the same is true for animals. In any given species, there will be great variability in their virtuousness, so to speak. Each individual has his or her own center of gravity with regard to competition and cooperation, which can even shift over time based on the reality on the ground. Natural selection works to favor one strategy over the other when the environmental conditions call for it. Meanwhile, mutation and sexual recombination keep supplying the diversity, so we always have some variability in just about all traits. Nature always stands ready to send a gentle species careening toward its darker instincts, should conditions require it.

As much as I wish this book could be nothing but rainbows and gumdrops, it is time to leave the comfort of the prosocial side of human nature and venture into the darkness of our antisocial side. Each one of us has our demons, as well as our better angels, because we are all a product of evolutionary forces that favored generosity in some instances and selfishness in others. Maybe by understanding our demons, we can disarm them.

DEFINING JEALOUSY

Jealousy is one of those human emotions that is not always easy to define without venturing into other emotional states like envy, bitterness, and even resentment. Yet we all know what jealousy is, even without having a handy definition. We know it, because we have felt it.

Merriam-Webster gives three definitions of jealousy:

1. (a) the intolerance of rivalry or unfaithfulness; (b) the disposition to suspect rivalry or unfaithfulness

2. hostility toward a rival or one believed to enjoy an advantage

3. vigilance in guarding a possession

The first two definitions seem to focus on people and relationships, while the third definition focuses on things. For the purpose of this chapter, I will stick to the first two definitions, those having to do with rivalry and unfaithfulness. The third use of jealousy, in my view, ventures into the territory of greed and envy, which will be covered in the next chapter. Having just finished the chapters on love, attachment, and grief, I think it is better to discuss jealousy as it pertains to personal relationships before we get into the issue of possessions.

I should also mention that the distinction between jealousy and envy has some regional differences. For example, it seems that in the United Kingdom, the distinction between jealousy and envy has more to do with whether you already have the thing or relationship in question. The definitions in the Oxford dictionary indicate that jealousy is the fear of losing people or things that you already have, while envy is coveting things that you do not have. I am not quarreling with the precise uses of the words “jealousy” and “envy.” After all, who am I to tell the English how to speak English? All I am saying is that in this chapter I will focus on the kind of jealousy that deals with relationships and individuals, not things.

For our purposes, jealousy encompasses at least three feelings: (1) the fear of losing a relationship, (2) the negative reaction we feel when a rival threatens to disrupt a relationship (or merely when we perceive such a threat), and (3) the continued anger and/or despair when a relationship is lost to a rival. Further still, the relationship under threat need not be sexual/intimate. We can be jealous of the love that a parent gives another sibling. We can be jealous of the attention that our best friend gives to a new friend. We can be jealous when our boss showers praise on a colleague. And, of course, we can be jealous when our spouse looks longingly at another. Jealousy is all of that and more.

SEXUAL JEALOUSY AND MATE GUARDING IN ANIMALS

The kind of jealousy that we probably think of first is also the easiest to understand biologically. In humans, sexual jealousy is the fear that an intimate partner is not being sexually faithful. Romantic jealousy is the fear that an intimate partner may fall in love with another and thus end the relationship in favor of a new one. These two can sometimes be distinguished from one another, just as sex and love can sometimes be separated. For example, some people tolerate some extramarital sexual activity by their spouse but would be severely threatened by the possibility of a spouse leaving the marriage altogether. Also, in open marriages, sexual monogamy is never expected, but the love and commitment of the marriage is generally understood as exclusive, and thus, romantic jealousy can still appear.

For simplicity’s sake, I will discuss the issue of sexual and romantic jealousy as one single topic. Most human marriages and intimate relationships operate that way, and I am not sure that they can be cleanly dissected from one another in animals, either. Furthermore, with humans, it is difficult to say what role culture has played in forming our understandings of faithfulness and jealousy, which may obscure the underlying biology. We are still not really sure what the underlying natural biological state of the human family really is or if there even is such a thing as a natural state. There is plenty of evidence that early human families were extended clans with communal parenting and little sexual monogamy. This is certainly how most of our primate cousins build families.

On the other hand, sexually exclusive binary marriage, though certainly not universal, is the family paradigm that is the most widespread in native cultures throughout the world. Even in those traditions and cultures that espouse plural marriage, sex outside of marriage is considered prohibited and would evoke jealousy from the cuckolded spouse. Meanwhile, still today in some Amazonian tribes, sexual fidelity is unheard of and ritualistic group sex occurs. And yet, families consist of male-female marriages and child-rearing. The point here is that it is very difficult to differentiate cultural underpinnings from biological ones when it comes to sexual and romantic jealousy. Our psychology is the result of both our biology and our social experience. I think the study of jealousy in nonhuman animals will help us sort that out.

In 2011, a pair of young Malayan tigers named Seri (female) and Wzui (male) were brought to the El Paso Zoo in the hopes that Wzui would impregnate Seri, as well as another female already living at the zoo, Meli, who had lost her mate to cancer. Malayan tigers are highly endangered, and zoos have begun breeding programs in which they periodically exchange tigers in an effort to promote genetic diversity. Little is known about the mating behavior of this species, and breeding them in captivity has proven difficult, given the very narrow fertile period of just three days each year.

Seri and Wzui appeared to form a mated pair very quickly with plenty of sex and social bonding. Meli, who was much older and had been at the zoo for ten years already, ignored the young lovebirds for the most part. However, after some months, the zookeepers noticed that Meli and Wzui began to “flirt” through the fence that separated them—that is, they began paying attention to one another and made the purring sound that is widely recognized as a sign of affection and friendly greeting among felines. This enraged Seri, and she began to make aggressive displays toward Meli, who responded in kind. As the animosity between the two females grew stronger, Wzui’s interest in Meli continued to develop, even though they were being kept in separate areas and could not touch each other. The zoo even put out a press release reporting that a tense “love triangle” had developed among the tigers.2

One day, after an affectionate morning of sex and grooming between Seri and Wzui, Wzui was spotted, once again, looking at Meli. Meli flirted back, and the two watched each other through the fence for a while. This was the last straw for Seri. In a surprise attack, she lunged at her mate, going directly for his throat. Wzui was killed almost instantly.

What else can we say but that Wzui was the victim of jealous rage? Seri wanted him all to herself, and if he would not stay faithful, he would pay the price. If Meli had not been in a separate pen, Meli and Seri probably would have fought openly. Frustrated that she could not attack the real target, Seri focused on the target that she could reach.

An even more bizarre incident of jealous rage in animals dates back to 1902 in a zoo in Marseilles, France. It seemed that the lone gorilla at the zoo, a male silverback named François, had become quite taken with his human handler, Journoux. It is not clear if this attachment was one of a friendly nature or of sexual lust. Although it seems odd, it could have indeed been sexual/romantic, given how long this silverback had been without the company of other gorillas. In any event, Journoux got married and brought his wife to see the zoo animals under his care. François was immediately uneasy. Somehow, he figured out that the relationship between Journoux and his wife was different than that of Journoux’s other relationships. This one was a threat to him. François became hostile to the interloping wife and also to Journoux himself. This went on for some time before Journoux attempted to resolve the conflict by approaching François alone and offering him comfort and consolation one-on-one. This well-meaning attempt ended with the violent deaths of both François and Journoux, each at the hand of the other.³

Tigers and gorillas are not the only animals in which we see outbursts of jealous rage. There is a species of marine crustacean called cleaner shrimp. These little guys are fiercely territorial and form pair bonds for mating that are remarkably long-lasting and monogamous, something that is almost unheard of for invertebrates. The pair bonding is enforced quite viciously.4 If intruders approach, the jealous mates chase them away, and if they catch the would-be interlopers, they often kill them. They have also been observed to kill their mates in response to infidelity.⁵ And now for the really weird part: these shrimp are all hermaphrodites. If you think we mammals have it bad, a hermaphrodite must watch its mate vigilantly because every single other member of the species is a potential rival.

It is sort of amazing that a fully hermaphroditic species evolved into a social structure that enforces monogamy. One would imagine that none more than hermaphrodites would tend to favor as large a number of mates as possible due to their need to promote genetic diversity. Also, there cannot be gendered behaviors among hermaphrodites, since there is only one sex. But there you have it: sexual jealousy among hermaphroditic shrimp.

Now a look at our fellow primates: The titi monkey, a little-known species of New World monkey related to capuchins, tamarins, squirrel monkeys, and howler monkeys, has a social organization that closely resembles the most common family structure found in present human culture. Male-female pairs mate for life, and the “pack” for these monkeys consists of the nuclear family only: parents and children. When the adolescents become fully mature, they leave their families, find mates, and start families of their own.⁶

Researchers at the California Primate Research Center performed a series of experiments with titis through which they probed the social behaviors surrounding monogamy. All of these studies revealed patterns of behavior that are eerily similar to those of human monogamy. One in particular probed for jealousy. The scientists took monogamous pairs and introduced potential sex rivals, placing them at varying degrees of proximity.7 The researchers then observed the reactions of the mated couple. Interestingly, the researchers found that the “married men” reacted to the approach of an intruder male by getting closer and closer to their “wives” and by displaying increasing amounts of aggression to the intruder. In other words, they acted a little jealous. Unmated males were largely indifferent toward intruder males.

Many baboon species have a social structure in which a troop consists of several males and several females, each with separate dominance hierarchies.⁸ Most of the time, sexual activity is common among all individuals in all combinations. However, things get stricter when the females are in estrus and males’ access to females becomes restricted. The females, in order of their dominance ranking, choose the males. Although females are born into their troops and never leave, males will roam about every few years, particularly when they are bumped from their places by young social climbers.

Once a male baboon has been chosen by a female and has mated with her, he will guard her jealously for the rest of her estrus. He will follow her around, restrict her movements, and become hyper-aggressive toward any male that approaches. He will also mate with her repeatedly over this time period. (You know, just in case.) So busy are the male baboons with mate guarding that, during the estrus phase, they will lose sleep and miss meals. They lose all trust in other members of the troop, including and especially their mates, and also including brothers and lifelong male friends. Trust and friendship are temporarily replaced with suspicion and jealousy.⁹

The interesting thing about these baboons is that all goes back to normal when estrus is over. No more mate guarding; no more aggression and hostility; no more jealousy. Females can go back to having sex whenever and with whomever they want. As such, jealousy is focused pretty clearly on the matter of procreative sex. The males protect their reproductive investment, pure and simple. By guarding their female mate, they ensure that the child that results will truly be theirs.

Paternal investment in offspring is an important consideration in baboons because, in most cases, offspring gain the protection of their father as long as he is certain of his paternity and remains in a relatively dominant position. On the other hand, when a new alpha male takes over, he sometimes kills all the small babies in the troop.10 He does this not only to eliminate competition for his own children but also in order to bring the nursing mothers into estrus again, so that they can direct their reproductive and maternal energy to his children, not their future rivals.

The mate guarding that occurs in baboons is common in other primates as well. In gorillas, the fertile period of the females is not as limited as in baboons. Probably for that reason, gorilla troops only consist of one adult male: the silverback. Somewhere in gorilla evolution, the extended female fertility and constant male mate guarding made it impossible for males to live together, and the harem social structure evolved as the only harmonious scenario. When one silverback is successfully deposed by an invading male, what do you suppose happens to the youngsters? You guessed it—he kills them all.¹¹

The London Zoo learned that lesson the hard way.¹² Their gorilla enclosure contained a small band of gorillas organized into a harem. In 2010, the silverback died suddenly from diabetes. As expected, the females became very agitated and anxious. The harem is part of their natural social state, and deviations from that cause great stress and a resulting decline in health. Females in a harem without a silverback will even pull their hair out. Making matters worse, one of the females at the London Zoo was about to give birth.

The zookeepers were faced with a terrible choice. If they were to introduce a new silverback, the infant gorilla, once born, could be killed by him. If they did not introduce one, the condition of the three adult females would continue to deteriorate and the infant could die from malnourishment anyway. He could even get caught in the crossfire between the agitated females. (Adult female gorillas are generally hostile to each other; their fragile peace is kept by the silverback.) The zoo chose to introduce a new silverback, Kesho, and resolved to monitor the situation carefully. Soon after, an adorable young male named Tiny was born.

To their credit, the zoo staff handpicked Kesho from all the silverbacks they had access to. They saw him as the most likely to make the transition peacefully. He was only eleven years old, well before his testosterone-peaking years. He was untried as a leader and somewhat submissive and bashful, hardly the brutish bully that we picture with silverbacks. The zoo staff kept the pregnant female in a separate enclosure and waited until Tiny was born healthy and Kesho had bonded with the mother through the fence to slowly begin introducing them. In short, the zoo staff had done everything they could. Besides, who could possibly harm a cuddly and innocent infant? It turns out that Kesho could. He savagely beat Tiny to death during their second closely supervised visit.

This is a tragic lesson that silverbacks are not at all interested in adoptive fatherhood. They know which children are their biological offspring, and those that are not are in grave danger. A male gorilla cannot waste the precious parental resources of his troop to bring up the offspring of other males. There is just no room for that kind of generosity in jungle life.

There is another feature of silverbacks that is a natural consequence of the harem social structure: they are unabashedly misogynistic. Silverbacks are well known to dote on their sons but not so much their daughters. After their sons have weaned, a silverback takes over the feeding responsibilities for the young ones. When he does so, he will ensure that all of his little sons get their fill before he offers any to his daughters. These hyperalpha males are fiercely protective of their sons and do not appear to give much regard to their daughters.13 Come to think of it, I know some hyperalpha male humans like this.

For a silverback, the ultimate priority is running the harem in a way that promotes the success of his genetic offspring, and children of other males cannot be tolerated. They favor their sons over their daughters because their sons will need to grow up big and strong in order to fight for the privilege to run a harem. Life for the young males will be difficult—few males survive to run their own harems. On the other hand, the silverbacks know that their daughters will be just fine. Female gorillas already have a good shot at passing on their genes without much fuss. The silverback does not need to dote on his daughters; it is the sons who are in a precarious position. There are always several females for every one male. Where are all the leftover males? Killed by rival males.

What does this say about jealousy? The gorilla harem is the ultimate expression of mate guarding. One male completely dominates several females to the exclusion of all other males. Any possible intruder male will be viciously attacked, and any children of the previous harem leader will be killed. This is sexual jealousy taken to its logical extreme. Thankfully, most human males do not commit jealous murder in their lifetimes. However, I bet most men have contemplated it, however briefly and unrealistically. Even if most would never do it, they would think about it—even fantasize about it. And what is fantasy if not entertaining the notions brought into our consciousness by instinct? Our “natural state” might be more murderous than we think.

SEXUAL AND ROMANTIC JEALOUSY IN HUMANS

Is there reason to believe that human jealousy is akin to the mate guarding that we see so commonly in nature? The picture is muddy because of the strong influence of cultural norms on gendered behaviors. The traditional view that, in animals, “males are promiscuous; females are coy” dates back to Darwin himself. This was likely a projection of Victorian sensibilities regarding human sexuality onto animals and is the one area in which Darwin very likely mucked things up. Earlier generations of psychologists and biologists operated under the assumption that men were more likely to engage in infidelity and become sexually jealous.14 As they say, a thief thinks everyone steals. However, more modern studies have revealed that sex differences when it comes to love, fidelity, and attachment are not as great as once believed. The pall of Victorian values on animal sexuality is beginning to lift.

In contrast to what was believed about the sexual jealousy of men, it was once believed that women were more likely to experience emotional jealousy toward their male partners. It was thought that women are more scared of their husbands enjoying the company of others—not necessarily other women—to theirs. This evokes a rather 1950s style of sexual politics. As I said, the gap between the sexes is narrowing. This could be because of the erosion of gender bias among scientists, or it could be that the culture really is changing right underneath us. I suspect both. The world is constantly changing. Both we, the subjects of study, and we, the scientists doing the studying, have changed in all ways related to gendered behaviors. Those changes are still underway, but we will proceed and explore what we know (or think we know) about the biology.

In species that engage in the harem lifestyle of one male dominating the sexual reproduction of multiple females, such as lions and gorillas, there tends to be large size differences between the sexes. Male gorillas are typically twice the size of female ones, for example. This is because dominance of a harem is typically won by fighting other males, often to the death. This violence-based system does not much hurt the overall reproductive potential of the species because females are the limiting factor in reproduction and the fighting rarely harms them. On the other hand, in species that are monogamous, males and females are typically the same size. Gibbons are a good primate example of this. Similarly, in species in which sexual freedom and promiscuity is so lush that sexual rivalry is nonexistent (like bonobos), size differences between the sexes also appear negligible.

Human males are, on average, larger than females. Does this mean that we are evolutionarily adapted to the harem lifestyle? I suspect not. First of all, the sex-size difference in humans is nowhere near as stark as it is in gorillas or lions. Furthermore, some human ancestors showed much greater size differences than we do. In other words, in our recent evolutionary lineage, we seem to have evolved away from size differences between the sexes. Even if the harem lifestyle was once present in our distant ancestors, all anthropological and archaeological evidence indicates that our species moved away from it long ago.

However, the question remains: are we wired for paired monogamy, as suggested by modern and historical culture, or for communal social units, as suggested by evidence of prehistoric human society? Although likely unanswerable, this is an important question because mate guarding could either be something that humans have been evolving away from or something that we have been evolving toward. Remember that the development and evolution of behaviors takes time. Any particular moment in time, including the present moment, is only a snapshot of a system that is in great flux with no particular target end point.

Further still, the historical male sexual dominance over women that, until recently, was widespread in human cultures is not the norm in other primates.15 Once again, it is hard to say if the historic human tendency for males to dominate the sexuality of females stems from biological factors or sociological ones. One thing is for certain: the gradual progression away from that cultural phenomenon is bringing relief and freedom to women. While we still have a ways to go, especially in some parts of the world, this liberation could rightly be described as an evolution—at least a cultural one.

We can ponder whether humans are built for monogamy until the cows come home (and I suspect we will), but there is no mistaking that humans are indeed prone to sexual and romantic jealousy. While I certainly do not quarrel with the notion that sociological factors play a large role in how this jealousy takes shape, I do think there is strong reason to believe that biology underpins some of it. When you were reading about the sexual jealousy of animals earlier in this chapter, did it sound very familiar?

Think back to the episode of Seri, the tigress who killed her husband with the wandering eye, or the hermaphroditic cleaner shrimp that viciously attack and kill any approaching rival. While these episodes may appear savage and brutal, are they so different from what happens when jealous humans lose control?

Jealous killings happen so often that there is a term for it: a “crime of passion,” although technically any rage-associated killing could also be classified as such. According to the U.S. Bureau of Justice Statistics, 30 percent of all female murder victims were killed by their husbands and nearly 20 percent were killed by their ex-husbands.¹⁶ Even when jealous killing does not actually happen, it almost does. There was a study of five thousand people, both women and men, from six different cultures, in which 84 percent of women and 91 percent of men admitted to having at least one fantasy about killing their lover or a romantic rival.¹⁷ Those are big numbers. This is why, when a married person is killed mysteriously, any detective will tell you that the first person the police need to rule out is the spouse. Of all the seven billion people on the planet, the one person that you are most likely to be killed by is your spouse. Sleep tight.

Furthermore, when it comes to jealously protecting and promoting paternity, humans are not that different than gorillas. You probably recoiled in horror a few pages ago when you read about Kesho viciously killing the infant Tiny. What Kesho did was terrible. Awful. Horrific. It was also perfectly in line with what just about every human civilization did when they conquered other civilizations. Whether the Babylonians, Mongolians, Greeks, Romans, Mayans, Visigoths, or Chinese, a pretty universal phenomenon of conquering peoples was to kill the men and children of the vanquished people and take the women as wives or concubines. This was part of how an empire integrated new people so quickly. It was not just cultural assimilation; it was genetic assimilation, facilitated by the murder of the men and their offspring and the rape and forced marriage of the women.18

You do not have to reference prehistory, classic antiquity, or contemporary hunter-gatherer tribes to find this barbaric behavior. Christopher Columbus “discovered” the New World just over five hundred years ago. Following this watershed historic moment came the systematic eradication and enslavement of the native peoples and cultures. However, they did not suffer genetic extinction. The entire ethnicity we know as mestizo, which includes the majority of Latinos, is largely the result of Spanish men fathering children with Native American women. Mestizos comprise the largest racial group in the Americas and one of the largest on the planet. This is just a recent example of the human predilection toward reproductive dominance.

When it comes to jealousy, humans really are not any more evolved than primates, and if the evidence here does not convince you, maybe some brain scans will. Scientists at Emory University have studied the brain activity of jealous monkeys using the technique of Positron Emission Tomography (PET) scans.¹⁹ Specifically, they monitored the brains of male rhesus macaques as they watched their female mates. Then, they introduced a male rival and observed how brain activity changed. PET scanning exposed which parts of the brain were active at a specific moment in time. This research revealed a neural pathway that corresponds with jealousy—the “jealousy program,” if you will.

Using this same technique, another research group went a step further and explored how the jealousy program operates in a monogamous species of primate, the titi monkey.20 Using these monkeys, the scientists were able to look for the jealousy program before and after pair bonding. Sure enough, the jealousy program only gets “installed” in the male titi monkey after he has formed a pair bond and is activated when a rival approaches his mate, not another female.

This study builds on the earlier work in macaques in two important ways. First, it clarifies the fact that pair bonding is what creates the jealousy program and that mate guarding is what activates it. Second, the examination of titi monkey brain activity shows that the jealousy software runs in the primate brain in similar ways in monogamous and nonmonogamous species. This is key. In my view, romantic jealousy is unique to monogamy, while sexual jealousy can be present in any sexual species. That the jealousy program is so similar in monogamous and promiscuous species argues that romantic and sexual jealousy are really the same response, or at least extremely similar ones. In terms of brain activity, there does not appear to be any distinction.

Since romantic and sexual jealousy may be the same thing, the distinction between them in humans is found in what “trips” the jealousy program to become active. For romantic jealousy, the activating event is a threat (real or perceived) to the emotional attachment; for sexual jealousy, the stimulus is a threat to the sexual monogamy. However, once activated, the jealousy software runs on the brain hardware in the same manner and produces the same results: protectiveness, guarding, anger, despair, and potentially violence.

To demonstrate this, scientists ran the jealousy experiments on humans. Although this study focused largely on discovering differences between men and women, scientists in Japan monitored the jealousy reaction in the human brain using PET scanning and found that it is remarkably similar to that of macaques and titi monkeys.²¹ What this means is that the jealousy program is ancient and hardwired into the human brain. Of course, various species will evolve differences in how the program is activated and what the output will be, but the program itself is shared.

Also, I should say that I do not dispute the role of social conditioning and culture on our experience of jealousy. Some people are clearly primed for dramatic jealous responses, while others seem to be oblivious of blatant infidelity, practically right in front of their eyes. I doubt that those differences are genetic. My point here is to say that jealousy is an innate feature of humanity and something that we share with other mammals.

THE ROOTS OF INFIDELITY AND SEXUAL JEALOUSY

Even for those that contend that binary parenting and the traditional nuclear family are the natural state for Homo sapiens, there is good reason to believe that a drive toward infidelity remains present. If that traditional view of marriage and family life were biologically correct, the evolutionary basis of the drive toward infidelity would be different, but similar, between the sexes. For an early human man, infidelity would simply be a way to achieve additional paternity. By sowing his oats in as many fields as possible, he has a greater chance of leaving offspring, and thus his genes for infidelity, in great abundance. The genes that underpin the drive to procreate freely and widely would tend to accumulate. It is simple math.

In early human women, however, the drive toward extramarital sex was probably a bit different. Unlike birds, they were not able to simply sleep around the village and drop their eggs off for others to rear and raise. Nest parasitism is not really possible for mammals. Any additional maternity that a female achieves through infidelity, she must also care for, at least during the gestation and nursing phases. As long as she has a mate, why would there be any reason to look for extramarital sex at all? She already has access to the means of producing offspring as fast as she can. She, not the males around her, is the limiting factor in her reproductive capacity. This simple logic has, in part, led to the Victorian belief that still pervades human culture that women have far less tendency toward infidelity than men. The problem is that this logic gets lots of the biology wrong.

Women do indeed have several reasons to want to engage in infidelity. First of all, while females of all species do their very best to select the best mate that they can, we all know that different individuals have different strengths. There is no one perfect man (or woman). As females (and males!) begin producing their offspring, there is a strong incentive to mate with several suitable mates in order to produce sons and daughters with great variety in their genetic toolkit. The best bet in ensuring the success of progenitors is not just by having lots of them but by having lots of kinds of them: tall, short, fast, strong, camouflaged, brightly colored, good climbers, fast runners, quick thinkers, good instincts, and so on. The best way for a female to achieve this diversity in her offspring is to mate with lots of different males, not just the one she may be life paired with. For her, it is not about quantity; it is about quality and diversity.22

There are other incentives that primate females have for engaging in infidelity. First, by mating with other males, females help to protect their offspring in the event that one of her lovers moves up the dominance hierarchy. As we have already discussed, when a new male takes over a harem (as in gorillas) or even when a new alpha becomes the head of a multi-male troop (as in baboons and chimpanzees), he often kills the previous alpha male and possibly some potential rivals, as well as any children from the previous alpha. However, a new alpha may show mercy to the children of a female that he has repeatedly mated with. After all, some of those children may actually be his. Because of this phenomenon, female infidelity is something of an insurance policy to protect one’s children from future harm when a new dominant male takes over.

Modern humans like to think that we are above all of this. So be it. My point here is only to say that there are plenty of explanations for intrinsic instincts toward extramarital sex in both sexes. Recent surveys have borne this out, showing that the incidence of, and temptation toward, marital infidelity is more or less the same between men and women.23 Interestingly, several studies argue that a woman is more likely to be unfaithful during her narrow fertile period around the time of ovulation.²⁴ Especially because ovulation is concealed in humans, this bears the mark of a deeply ingrained subconscious instinct: “I’ve got a good mate, but now’s my chance to get some more diversity in my clutch—and maybe even some better genes.” Remember, the evolutionary drive is not just to have children, but to have successful children.

The concealed nature of human female ovulation is peculiar indeed. In the other apes, the fertile period of a female is rather obvious. Their genitals become engorged and red. You can spot them a mile away. This is very useful for the husbands because they know when to mate with and guard their wives. As mentioned earlier, in baboons, mate guarding only takes place in the fertile period. After that, the males do not bother to guard their mates.

In contrast, human females do not advertise their ovulation at all. It is totally obscured, even to themselves. Why? This seems imprecise and inefficient. Without knowing when she is ovulating, a male would have to mate with her throughout the year and guard her constantly…bingo! This is the leading hypothesis of why ovulation is completely cloaked in humans.²⁵ The uncertainty about the timing of ovulation keeps the men interested in the females all the time. A male needs to stay close by to keep other males away, and he needs to mate with the female frequently to increase the chance of hitting the reproductive jackpot. This frequent mating strengthens the pair bond. Could concealed ovulation have been crucial to the genesis of the human family as we know it?

Not so fast. Concealed ovulation also means that adulterous suitors will be constantly interested as well. Since they do not know exactly when she is fertile, any quick dalliance is worth a try, and remember, she has incentives to get a little on the side. She could be protecting her children from a potentially murderous future alpha, or she could be shopping around for better genes or more diversity among her children. Either way, by hiding her ovulation, she keeps them interested.

Why all of this talk about infidelity in a chapter about jealousy? Key to the modern experience of jealousy is that both men and women have the demon inside of them, driving them to seek extramarital sexual activity. There is little to be gained by pretending that is not true. Many have conquered that demon by the sheer will of their conviction. Others have chosen not to fight it at all and instead choose a bachelor(ette)’s life or an open marriage. The great majority of us, however, are stuck in the middle somewhere, doing our best to maintain our promises of fidelity in the face of temptation, while also suspiciously guarding our spouses from those same temptations. In other words, we are at war with ourselves and with each other and therefore doomed to live jealous lives.

We humans are utterly convinced that we have evolved the ability to make our own choices and control our own destiny. We can choose our mates, our lifestyle, our commitments, and our fidelity (or lack thereof). Because of that, we like to think that our relationship-based emotions have become too sophisticated for an overly simplified look at jealousy. In humans, we tend to think that romantic jealousy must be much more than merely an attempt to protect reproductive investments.

Some data argue that we may be just as simple as our fellow primates. Researchers at Northern Illinois University conducted a study of many hundreds of college-age students.26 Some were in relationships; some were not. Some were gay, lesbian, or bisexual; others were straight. Respondents were walked through several hypothetical scenarios and then asked to report how jealous they would feel in those situations. There were three hypothetical people: the respondent was the person participating in the survey, the partner was the intimate partner of the respondent (real or imagined), and the rival was the person tempting the partner into an illicit sexual affair. The survey involved scenarios in every possible direction: a single straight male whose imaginary girlfriend was being seduced by another man, a partnered gay man whose boyfriend was being seduced by a woman, a lesbian being seduced by another woman, and so on. You name it, they tested it, and respondents were asked to rate their jealousy and describe it as sexual versus emotional (romantic) in these various circumstances.

The surveys revealed that sexual jealousy spiked most when a woman was tempting a man or a man was tempting a woman, regardless of the sexual orientation of the threatened party. When it came to romantic jealousy (called emotional jealousy in this study), the results were more in line with what we would expect. The lesbians were threatened by the advances of a female interloper, the straight women by the intrusion of another straight woman, and so forth. The study also found what has previously been reported many times before: heterosexual females are more susceptible to romantic jealousy and less susceptible to sexual jealousy than heterosexual males (although this apparent phenomenon may have been erroneous all along, owing more to how women and men view the question differently rather than innate differences in how jealousy is activated.27 Clearly, more research is needed.)

At first, I found this result odd. One would assume that a lesbian would always be most threatened by the attempted seduction of her wife by another woman. I would have suspected that flirtatious men would not bother her at all. When it comes to emotional jealousy, they do not, but the element of sexual jealousy is still there. Why? Perhaps it is because she knows that, technically, her wife and the intruding male could make a baby together. That possibility is unsettling to contemplate. Regardless of the emotional connection (or lack thereof) of an affair, the thought of our partner engaging in extramarital sex with a potential future co-parent boots up jealousy programming in our brain. Underneath the complexities of our emotional experience, reproductive potential is still an important aspect of human sexual jealousy, regardless of gender or sexual orientation.

But why should we have such great concern about reproductive fidelity in our mate? For males, it is rather simple. He does not want his mate(s) spending time, energy, and resources gestating, birthing, and caring for other males’ children. Any infidelity by her is a direct and potent threat to his reproductive success, plain and simple, because it takes time and resources away from his children. He has a strong reason to try to stop it, and jealousy is the drive to do that.

For females, things are a little more nuanced. Why would she care about her man sleeping around? Once again, it comes down to his investment in the success of her children. This is where species differences in reproductive behavior come into play because some fathers, such as humans, invest a great deal in their children’s success, and in other species, males do no parenting whatsoever. For species that do show paternal investment in child-rearing, the females know that if their mates achieve outside paternity, his protection, resources, and so on will then be split among her children and those of some other female. Even in the harem lifestyle, where a male has constant unhidden sex with multiple females, there is extensive tension and jealousy among the common mates of a single male. In gorillas, this is part of why the silverback is necessary to keep the peace among them. Interestingly, if the “sister wives” are actual biological sisters, as is usually the case in lions, jealousy is not a problem.28 This makes sense because sisters share genes; helping your sister is almost like helping yourself, genetically speaking. If the wives are not sisters—well, remember Seri, the tigress and jealous wife who killed her husband with the wandering eye?

In sum, sexual and romantic jealousy, like so much else about our nature, is about reproduction. It is a drive, an instinct, to defend our reproductive success and prevent our efforts and resources going to the reproductive success of others. You might now ask, “Yes, but what about those who have chosen to forgo reproduction or who are beyond reproductive age? Those people still get jealous. Why might that be?”

Keep in mind that our drives and tendencies are largely subconscious. Behavioral drives are messy, imprecise, and immune to being neutralized by other facts on the ground. Although conception for procreation is not the only function of sex in many species, it is nevertheless a key focus of the sex drive. And yet, if you vasectomize a male, his sex drive is not affected in the least. Similarly, when a young married couple uses artificial birth control, the sex drive does not “know” and is as strong as ever. It is the same for jealousy. Just because a couple has chosen a child-free life does not mean they will be free of sexual and romantic jealousy. We cannot simply flip a switch and deactivate the jealousy program that is written into our mammal brains.

In sum, human sexual and romantic jealousy, while different in context, both harken back to mate guarding and defense of reproductive investments. As such, jealousy is one of our more basic emotions, whether we like it or not.

SOCIAL JEALOUSY IN ANIMALS

The biological forces underneath sexual jealousy seem pretty straightforward. Males do not want to suffer reduced paternity or spend time and resources on other males’ offspring. Females do not want the father of their children spreading his attention beyond the homestead. On the other hand, jealousy of other kinds of relationships—friendships, for example—seems wholly different. That kind of “social jealousy” is not found in the animal world, is it?

Of course it is. It is common knowledge that dogs get jealous very easily and very often. Any time a dog owner gives attention to another dog, there will be a jealous reaction. It may involve sulking, ignoring, or even depression, but more likely, the jealous dog will become aggressive toward the interloper. My own dog, Bruno, is a perfect example of this. When I would go into my former roommate’s bedroom to pet her cats, Bruno would sit in the doorway and watch, clearly agitated. As I pet the cats, he would whimper and whine and periodically jump at the gate. If I took things further and uttered an enthusiastic, “Such a good boy!” to the cat, this was more than poor Bruno could take. He would go into hysterics, jumping and barking and howling. In short, he flew into a jealous rage.

Social jealousy has been observed in just about all companion animals. Whenever they perceive their owners giving attention to a possible replacement or rival, jealousy is usually the result. With sexual jealousy, animals and humans are responding to the very real danger of reduced reproductive productivity, but with seemingly innocuous social threats, why the obnoxious response?

To understand this, we must look at relationships in the animal world with an ice-cold eye. Whatever else animals feel, whatever attachments they form, and whatever grief they experience, they view their social relationships as commodities. For a given animal, his relationships have real value for his fitness—his survival and reproductive success. Animals make friendships and alliances that benefit them. Pair-bonding species select their mates carefully based on “what is in it” for them. Even the relationships between parents and children can be viewed in terms of value—the value added for survival and reproduction.

I know this is a bucket of cold water poured over our warm, fuzzy feeling toward animals, but animals really do approach their relationships as merchandise, as tools to help them succeed. Some species drop their mates when a better one becomes available. Friendships and social alliances are disrupted as the young and ambitious move their way up the ranks. Parents can abandon or even kill their own children when they become a liability in some way. For example, if a burdensome child is consuming resources that could go to healthier or more socially successful siblings, what is a parent to do out there in the harsh world?

With this in mind, social jealousy makes perfect sense. It serves to protect relationships that are of social or reproductive value. Take dogs, for example. Whether they view themselves as alpha or they are submissive to someone else, social rank is important. Your dog is jealous when you give affection to another dog because she is fearful of losing her place in the social structure. Each relationship has value in the dominance hierarchy, and rank matters to her. This is because higher-ranked wolves are more reproductively successful, dominate the food resources, and so on. It does not matter that your dog is spayed and has no shortage of food—the drive to jealously guard social status is deeply ingrained.

Accordingly, dogs view various relationships differently. It is not uncommon for dogs to favor some family members over others. Further, when strangers are introduced (human or animal), the same dog may react warmly to some but be hostile to others. It is not just unfamiliar individuals who evoke jealousy. Many a budding relationship has been quashed because the dog simply did not like the new girlfriend. Furthermore, in multiple-pet families, some dogs form close alliances, while others merely tolerate each other (on good days). Once again, these differences are usually best understood by viewing each relationship as a commodity. Dogs continually but subconsciously evaluate what each person or pet provides.

For example, if a dog recognizes a clear alpha in the house, she often attempts to maintain a close alliance with that person (or dog!), unless that individual rebuffs the attempt or repeatedly harms her. For better or worse, some breeds of dog respond to tough corporal punishment with staunch loyalty. This is not because they are masochistic. It is because they know that it is good to have powerful friends.

The one exception to this rule is the fact that many dogs are gentle and attentive with young human children. This would seem to be a conundrum because what advantage could it bring to befriend the weakest member of the pack? This is probably a quirk of dogs. Remember that their genetic behaviors are not solely those that were inherited from wolves. They are also the product of recent intense selection that honed certain work behaviors for the benefit of their human domesticators. Dogs have been bred with instincts and abilities to perform trained tasks, including herding, guarding, and protecting. This ingrained some special protective instincts toward little ones. (Also, I think it is pretty obvious that dogs know that kids are a ready source of dropped food. There is little to be gained from biting the hand that feeds you.)

SOCIAL JEALOUSY IN HUMANS

The value-threat interpretation of social jealousy translates well to all social species, including humans. We, like dogs, feel threatened when someone encroaches on a relationship that we value. For relationships that we do not value that much—eh, who cares if they went out with other friends and neglected to call us. Dogs do not show much jealousy if a member of the pack that they are not closely bonded with shows affection to another dog or human. If you scan the discussion in the last few pages about social jealousy in animals, you will probably not struggle to imagine the human parallels.