Biology is not fully reducible to genetics, and behavior is not fully reducible to biology. I’ve found myself using that adage more often in conversation, so I thought perhaps it would be helpful to mention and unpack here. As far as I know, that particular phrasing is mine, but the sentiment I believe is fairly representative of what biologists think. While there may be a popular expectation finding genes for every personality trait, the reality is that genes can only have so much influence. That should be apparent even from the raw numbers. We have roughly 25,000 genes and 100 trillion neural connections; so few genes cannot possibly fully specify how our brains are wired, especially considering everything else those genes also have to do. Even when genes get a label like the “warrior gene” the most we can about it is that it is associated with a higher tendency toward certain behaviors, rather than being fully deterministic.
Going a little further, what do I mean that biology is not fully reducible to genetics? I mean that not everything that happens in our bodies is fully determined by our DNA. We recently saw how prolonged time in space can cause differences between the biology of an identical twin and his Earthbound brother. We know the sex of some fish and reptiles is influenced by temperature, and a recent paper showed that turtle embryos may themselves have a mechanism for regulating temperature to achieve a specific outcome. We cannot perfectly predict traits from height to heart disease risk simply by reading one’s genome. Sure, eye color and blood type have fairly simple and deterministic genetics, but those are likely the exceptions. They are useful examples for teaching the basics of inheritance, but if they are all one knows one might have a skewed expectation of how genes relate to traits.
That biology does not fully determine behavior should be even more obvious. Typing this blog post is not an activity that flows purely from my biology. I had to learn a language, learn keyboarding skills, and learn biology, while others had to invent the Internet and write blogging software. Culture and experience clearly shape behavior. Of course, biology plays a role as well; we wouldn’t be quite so susceptible to certain clickbait headline techniques if our neural biochemistry worked differently. Behavior is a stew, a rich and complex product of the potential for myriad interesting interactions born of mixing multiple ingredients.
That context is helpful to bear in mind when reading the latest news about genetics and behavior, such as a study published last week on genes and sexual orientation. The study itself is an interesting byproduct of the Human Genome Project, cheap sequencing technology, a desire to know where we came from, and high expectations that to know one’s genes is to know oneself. Researchers had access to hundreds of millions of genomes thanks to 23andMe, a commercial personalized genome sequencing service; this allowed for a much larger study on the genetics of orientation than any previously conducted. By analyzing those genome sequences alongside survey results about behaviors, they could ask whether there is any evidence of specific genes for homosexuality. The results were consistent with earlier, smaller studies: some association between genetics and homosexual behavior exists, but genes do not fully predict such behaviors and the associations that were found are complex and involve many genome locations rather than a single gene. Thus orientation is likely neither a fully willful choice nor a genetic imperative.
I’ve covered this topic before, and came to a similar conclusion then. I explored some additional research which suggests there may be additional biological components beyond genetics, but even accounting for all of one’s biology there is still variability in orientation. Given that those posts are still read with some frequency and in light of the recent study making headlines, I thought it was a good opportunity to revisit the topic. As then, I still think it is important to remember that even our best understanding of biology can only tell us what is, and not what ought to be. And if we are going to hold that perspective, we need to be consistent. It can be tempting to appeal to biology as a model of how things should be when we think biology happens to agree with us, but bear in mind that the world of living creatures is wild and woolly and still has the capacity to surprise.