If I were to ask you the simple question,"Do you think that genes influence your personality?" the first thing you might think is that I'm asking you a stupid question. After all, nearly all our lay beliefs about the world include beliefs that some of our genetic material influences who we become as people. And though we do believe, to varying degrees, that our experiences shape who are, I'm sure we can't think of all that many people who believe, like Aristotle, that we are a tabula rasa (blank slate). As well, if you believe in evolution then you must have an implicit belief that genes influence who we are. If evolution has taught us anything, it is that survival means passing on the fittest of our genes to the next generation.

So, you come to this post with the implicit belief that your personality is most certainly influenced by your genes. What if I told that this is not what the most recent research in behavioral genetics would suggest?

Genes and Personality: The Early Years
In the early years examining the links between genes and personality, it was typical for a study to examine self-reports of personality and compare the self-reports between fraternal twins—who share roughly 50% of their genes—to those of identical twins, who share 100% of their genes. In these early twin studies, very consistent effects emerged that suggested one thing: when it comes to personality, genes matter.

In that work, researchers calculated heritability estimates—in lay terms, the amount of variation in personality that is explained by genes—by examining personality similarity between twin pairs. For identical twins, heritability estimates hovered around 46%, and 23% for fraternal twins (a heritability of 1.00 means that all variance is genetic; Jang et al., 1996). Together, this early work was very clear in its suggestion that there are some genetic influences on personality. The next question, was of course, which genes would be the biggest players in the gene-to-personality pathways?

Candidate Genes
The early work in twins is suggestive of the possibility that eventually, with enough knowledge about human DNA, scientists will be able to discover a specific gene for, well, for anything related to personality, preferences, intelligence, or physical characteristics. That's a potentially exciting domain of future research, and one that researchers have examined very vigorously in the last 15 years or so. In this work, affectionately referred to as "gene for" studies by one of my colleagues, researchers looked for specific small repeating sections of genes (single nucleotide polymorphisms or SNPs) that identified a version of a specific gene. The SNPs usually were related to the specific production or reception of neuropeptides implicated in any number of social behaviors in non-humans. One really famous SNP is the APOE4 genetic polymorphism, which has been linked to increased risk for Alzheimer's Disease in humans. Another one is the GG variant of the oxytocin receptor gene rs53576, which is associated with increased oxytocin receptors in the brain.

The critical point in these "gene for" studies is that, if we know what parts of personality that a specific neuropeptide influences, then its genetic variants should predict behavior in a similar fashion. More specifically, knowing how oxytocin influences personality (although oxytocin's influence on behavior is still in question) would suggest that knowing variations in specific SNPs on the oxytocin receptor gene should help us predict personality.

In the subsequent "gene for" research, many researchers were left disappointed Specifically, for every breakthrough finding linking a specific SNP to a personality characteristic, there was a null replication. Several of the most promising candidate genes, such as the MAOA gene which has been linked to antisocial behavior in past research (Caspi et al., 2002), have failed to replicate in subsequent work, according to several meta-analyses (De Moor et al., 2010).

So, then genes don't influence personality?

The current prevailing genetic evidence seems to suggest that we actually don't have genes for personality. And this conclusion doesn't come from a lack of trying: The US government has spent billions on genetic research. Billions. BILLIONS!!! When I think about all the money that went into this "gene for" research, I want to throw myself out the second floor window of the psychology building. The fall wouldn't kill me, but I imagine it would hurt just as bad as it does to realize that much of our research funding was flushed down the "gene for" toilet.

Of course, the conclusion that genes don't influence personality is most certainly wrong, after all, we have decades of twin research showing similarity in personality between identical twins. At least some of that similarity has to be genetic. Are we missing something that might help uncover the great mystery linking genes and personality?

Take a longer look at the genes.
One potentially promising approach involves examining many candidate genes that relate to a specific biological system associated with personality. In one such approach, Jamie Derringer led a consortium of researchers in an examination of a collection of SNPs associated with Dopamine in prior research, and then examined associations between this collection of SNPs and sensation seeking behavior. Sensation seeking is a personality trait that is linked to a number of behavioral disorders relating to substance use and addiction—and much of the human and non-human research indicates that dopamine plays a role in this behavior.

This work differs from the "gene for" research of the past because it doesn't rely on the association of a single SNP related to dopamine influencing sensation seeking. Rather, the study looks at a number of SNPs related to dopamine in prior research, to determine if these SNPs work in concert to influence dopamine levels, and sensation seeking more broadly. This approach is appealing because it involves conceiving of genes and personality not as simple one-to-one relationships, but instead, as complex systems of genes that work in concert to express a personality trait.

The findings of this research were promising: Taking into account all the SNPs associated with sensation-seeking behaviors as an aggregate, dopamine genes worked in concert to explain around 6.6% of variation in sensation-seeking behavior (Derringer et al., 2010).

We're still not there yet.
Remember that twin studies suggested that 40% of identical twin personality was genetic? Well 6.6% in the dopamine genes study is a far-cry from 40% in this twin research. Where does the rest of the heritability go?

One possible answer arises from understanding what happens to DNA before it is expressed as a personality characteristic. As your high school biology instructor will tell you, DNA is a code for building proteins, hormones, and neuropeptides that serve specific cellular functions within the body. One thing that early gene-personality work overlooked is that a lot has to happen to allow DNA to code for specific hormones/neuropeptides, that then have to act at the cellular level to subsequently influence personality. In short, genes need to be expressed at a cellular level in order to influence personality, and so one place where a genetic researcher might want to look to examine gene influences on personality is at this expression—that is, what genes are being unzipped by RNA, so that specific hormones/proteins are produced?

Research in honey bees is suggestive of the potential of examining RNA to predict behavior. In this work, messenger RNA abundance was a significant predictor of behavioral transitions of honey bees from hive workers to foragers (Whitfield et al., 2003). Human work in this domain is an exciting area of future research.

If you've made it this far, you can appreciate (like I do), that the question: "Do genes influence personality?" cannot receive a simple answer. On the one hand, genes clearly seem to contribute to personality, but on the other, much of the genetic evidence has not supported this view. I'm cautiously optimistic about the future of gene work. Are you?

Whitfield, C. (2003). Gene Expression Profiles in the Brain Predict Behavior in Individual Honey Bees Science, 302 (5643), 296-299 DOI:10.1126/science.1086807 

Derringer, J., Krueger, R., Dick, D., Saccone, S., Grucza, R., Agrawal, A., Lin, P., Almasy, L., Edenberg, H., Foroud, T., Nurnberger, J., Hesselbrock, V., Kramer, J., Kuperman, S., Porjesz, B., Schuckit, M., Bierut, L., & , . (2010). Predicting Sensation Seeking From Dopamine Genes: A Candidate-System Approach Psychological Science, 21 (9), 1282-1290 DOI:10.1177/0956797610380699 

de Moor, M., Costa, P., Terracciano, A., Krueger, R., de Geus, E., Toshiko, T., Penninx, B., Esko, T., Madden, P., Derringer, J., Amin, N., Willemsen, G., Hottenga, J., Distel, M., Uda, M., Sanna, S., Spinhoven, P., Hartman, C., Sullivan, P., Realo, A., Allik, J., Heath, A., Pergadia, M., Agrawal, A., Lin, P., Grucza, R., Nutile, T., Ciullo, M., Rujescu, D., Giegling, I., Konte, B., Widen, E., Cousminer, D., Eriksson, J., Palotie, A., Peltonen, L., Luciano, M., Tenesa, A., Davies, G., Lopez, L., Hansell, N., Medland, S., Ferrucci, L., Schlessinger, D., Montgomery, G., Wright, M., Aulchenko, Y., Janssens, A., Oostra, B., Metspalu, A., Abecasis, G., Deary, I., Räikkönen, K., Bierut, L., Martin, N., van Duijn, C., & Boomsma, D. (2010). Meta-analysis of genome-wide association studies for personality Molecular Psychiatry, 17(3), 337-349 DOI: 10.1038/mp.2010.128

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