Epilogue

WHAT DOES THE SCIENCE OF HUMAN INDIVIDUALITY tell us about free will and human agency? Are we genetically predetermined automatons, directed by our gene variants to have particular maladies, personalities, skills, intelligence, and sexual desires? Or are we pure blank slates, ready to be individually formed by our social and cultural experiences into shining creatures of free will, with limitless potential and choice? Of course, the answer is neither one. As we’ve discussed, a good phrase to replace the tired and inaccurate “nature versus nurture” is the more complicated “heredity interacting with experience, filtered through the inherent randomness of development.” Experience, in this sense, is a broad category that includes social and cultural influences, but also the illnesses you’ve had, your physical environment, the bacteria that have colonized your body, and even potentially the cells from your mother and your older siblings (and, for some women, the fetuses you have carried) that may still live in your body.

Some human traits are highly heritable and, of those, a few derive from variation in a single gene (like earwax type) or a fairly small number of genes (like eye color), but most others are polygenic (like height) and so reflect the interaction of variation in hundreds of genes. Yet other traits have little or no heritable component at all (like political beliefs and speech accent, respectively). Most traits, whether they are behavioral (like extraversion or fluid intelligence) or structural (like BMI or propensity for heart disease) come from a mixture of heritable and nonheritable factors. Behavioral and cognitive traits are highly polygenic, so there’s no single gene variant that accounts for shyness or creativity or aggression or ADHD.

Importantly, heritable and nonheritable factors can interact. This interaction can occur in a simple way: to get the disease PKU, you have to both inherit two broken copies of a gene for phenylalanine metabolism and eat foods that contain phenylalanine. Genes and environment can also interact through behavior. For example, if you’re born with gene variants that make you a fast runner, then you are more likely to engage in the sports for which this is an advantage, and that practice will make you even better at your chosen sport. The central point here is that genes and experience do not always work in opposition—rather, in some cases, they can reinforce each other.

Overall, adults in the United States are pretty good at estimating the heritable components of traits. In one recent online survey, most people guessed more or less correctly that, for example, political beliefs have a very small heritable component, height is strongly heritable, and musical talent falls in the middle. There are a few traits for which people’s estimates tend to be inaccurate. For example, most people think that variation in sexual orientation is about 60 percent heritable, whereas it’s really only about 30 percent (about 40 percent in men and 20 percent in women). On the other side, most people think that variation in BMI is about 40 percent heritable, when it’s really about 65 percent.¹ It’s interesting to imagine the ways in which cultural ideas inform these mismatches. In the case of BMI, I imagine that many people want to believe that food consumption is more a matter of personal willpower than it really is. This is a frequent, if mysterious theme. In most cases—from the accuracy of memory to the heritability of personality traits—people imagine that they (and others) have a greater degree of autonomy and personal agency than they really do.

THESE DAYS, THERE’S A lot of excitement and discussion about the prospect of using gene-editing techniques (particularly one called CRISPR-Cas9) to reverse, in embryos, certain genetic diseases that are produced by variants in one or a small number of genes. In 2018, He Jiankui, of China’s Southern University of Science and Technology, reportedly violated institutional approval and informed consent procedures when he deleted the CCR5 gene in human embryos. The embryos were then implanted into their mother, resulting in the birth of twin girls named Lulu and Nana. Ostensibly, the medical justification for this procedure was to ensure that the embryos would not become infected by HIV, which their father carried. In the absence of the protein directed by the CCR5 gene, HIV cannot gain entry into immune cells to infect them. He has been broadly condemned for this experiment. In addition to the issues of approval and consent, there is concern that CCR5 deletion will have unintended consequences for the gene-edited twin girls. We know, for example, that CCR5 is expressed in the brain, but its function there is poorly understood. It’s entirely possible that there will be neuropsychiatric changes that result from CCR5 deletion.

So, in addition to correcting genetic diseases or preventing infection, would it be possible to use CRISPR technology to change other traits? The answer is yes for traits specified by one or a few genes. For example, it would not be a technical challenge to ensure that your child had the variant of the ABCC11 gene that conferred wet earwax and stinky armpits. Eye color, which is mostly controlled by two genes (but which shows minor effects from fourteen more) is another human trait that, with a bit more effort, could potentially be manipulated by gene editing. However, as with CCR5, even manipulating small numbers of genes may produce unintended effects. For example, the variant of ABCC11 that confers wet earwax has been suggested to confer a slightly higher risk of breast cancer.²

Most of the traits that people would like to enhance in their children—like height, athleticism, and intelligence—are highly polygenic. In addition to important ethical considerations, that’s a technical problem on several levels. First, it’s not feasible to edit, for example, all one thousand or so presently known intelligence genes (which together only account for about 30 percent of the variation in IQ test score). Second, because these mutations do not just sum up, each adding a tiny bit to intelligence, it’s not necessarily clear how to produce the best combination. A variant of gene X may be associated with increased intelligence, and another variant of gene Y might be associated with increased intelligence, but when the two are expressed together, something unpredictable could happen. The double variant could reduce intelligence, or increase intelligence but produce epilepsy, or even create some medical problem that’s unrelated to the nervous system. Multiply that issue by a thousand and you see the scale of the problem. Our present state of genetic knowledge is such that it’s much easier to break a sought-after polygenic trait than to enhance it. At present, we know of a few single-gene mutations that will confer intellectual disability, but none that will strongly enhance intelligence.

WHEN YOU THINK ABOUT the individual genetic and developmental differences that impact the sensory portions of our nervous systems, it’s remarkable that we can agree on a shared reality at all. You’ll recall that 30 percent of the four hundred or so olfactory receptor genes are functionally different when comparing two random individuals. That’s the first step of the sense of smell, before we even consider individual differences in the brain circuits that process that information or the ways in which those brain circuits are changed by experience. Due to these innate and learned differences in smell and taste perception, my integrated flavor experience of Barolo wine or Cheez Whiz is not exactly the same as yours.

Crucially, these individual differences in perception are present for all sensory systems, not just smell and taste. My red is not necessarily your red, my G-minor chord is not your G-minor chord, and my chilly bedroom is not your chilly bedroom. This individual variation doesn’t hold just for those senses that point outward, but also for those that point inward and inform us about the state of our bodies. In this spirit, my sensation of a full stomach is not your sensation of a full stomach and my ten-degree leftward tilt of the head is not your ten-degree leftward tilt of the head. Each of us operates from a different perception of the world and a different perception of ourselves.

A portion of the individual variation in sensory systems is innate. But those innate effects are elaborated and magnified with time as we accumulate experiences, expectations, and memories, filtered through and in turn modifying those very same sensory systems. In this way, the interacting forces of heredity, experience, plasticity, and development resonate to make us unique.