Regarding Turkheimer’s Proclamation

Scientists in the field of behavioural genetics have long dreamt of discovering the genetic aetiology of particular behavioural traits, such as assertiveness. To be precise, behavioural geneticists were not interested in decisively vanquishing nurture from the perennial nature versus nurture debate, but they did want to show that genetics does play an important role in behaviour, and ideally demonstrate which alleles are causal factors in particular traits. After fifty years of research, the results are in; however, the body of knowledge cannot be understood without appreciating its relationship to entrenched political beliefs that are sometimes so painfully sensitive that they cannot even be touched. After all, behavioural genetics directly challenges the very mechanism upon which social constructionists have based their dreams of recreating a better society devoid of prejudice. Fortunately the problem lies in the mechanism and not the dream.

Social constructionism grew out of radical left-wing thinking in the 1920s, partly as a reaction against eugenics, and was formulated with a model of human nature that made racism and sexism as untenable as possible (Pinker, 2002). On the other hand, although behavioural genetics did not gain recognition until the 1960s, it really dates back to Sir Francis Galton’s Hereditary Genius (1869), making behavioural genetics the natural enemy of social constructionism. As a pioneer of eugenics, Galton’s name and theories are inextricably linked to a dark thread in history. If there is a lesson to be learned from this, it is that one should be skeptical of any scientific or intellectual theory which purports to remake society into something better. After all, we do not know what we do not know. Both eugenics and social constructionism share this property. Perhaps it is an indelible quality of human nature that social constructionism fails to raise the same red flag that eugenics ideally should have.

As the star of social constructionism rose and came to dominate the social sciences, behavioural geneticists were busily conducting studies and refining their techniques as they responded to criticism. By 2000, Erik Turkheimer, a behavioural geneticist, felt able to declare that the nature-nurture debate was over. An audacious and inflammatory claim, but one nonetheless well researched, precisely thought out, and backed by a vast body of mature, if unpopular, evidence. Turkheimer declared that everything is heritable. Not to put too fine a point on it, this is not to say that nurture plays no role, but rather that nurture’s role falls short of accounting for 100% of the variation in almost any trait of interest. The exact amounts have been codified with surprising robustness. Between 25% and 75% of the variance of behavioural traits are accounted for by genetics. A useful rule of thumb is that genetics accounts for 50% of the variance of a given trait (Pinker, 2002).

Turkheimer (2000) went on to summarise the main results of his field in “Three Laws of Behaviour Genetics.” The implications are profoundly important for understanding human behaviour and are particularly relevant to understanding childhood development, and how we come to have the personalities and behaviours that we do. The first law, as already stated, is that all human behavioural traits are heritable. This is not to say that everything has been tested and is already known, but rather, that it seems no matter what you study, heritability plays an important role. This has nothing to do with causation, per se, because genetics and environment share a complex non-linear interaction. For example, both height and attractiveness are heritable, and attractive tall people have a privileged advantage within society. People think that attractive people are nicer and more intelligent, and on average, tall people receive promotions earlier (Pinker, 2002). It is possible, in theory, that this social advantage may lead tall attractive people to behave more assertively. So it is plausible that alleles affecting attractiveness and height may have some role in assertiveness itself. But it is absurd to think that genes for attractiveness and height cause assertiveness. The pathways from particular genotypes to particular behavioural phenotypes are exceedingly complex and not amenable to experimentation, so the relationship between genetics and behaviour remains mysterious. However, the first law is that genetics seems to play an important role in every behavioural trait.

The second law states that “the effect of being raised in the same family is smaller than the effect of genes” (Turkheimer, 2000, p. 160). Simply put, on average parents have a negligible effect on the long term behavioural traits of their own children. It is well established that roughly half the variance in any given trait comes from environmental and random factors (2000). A portion of that environmental variance is shared between siblings. They live in the same house and street, with the same parents and neighbours, and all of the associated benefits or deficits in money, home life and importantly, parenting philosophy and technique. However, it seems that 0% (in certain cases 10% at most) of the variance in behavioural traits can be linked to this shared environment. For example, siblings are no more similar or different if they are raised together or apart (2000). Whatever effect the environment has on children’s behaviour, it is their unique experiences that are important, and not parenting philosophy (2000). This is not to say that damage cannot be done by abuse or neglect, but that rather, when you examine parenting across the population within a given culture, the individual differences in parenting philosophy and technique account for little if any of the variance in particular behavioural traits. For example, if two authoritarian parents raise four authoritarian children, it is unlikely that those children became authoritarian because of any systematic behaviours of their parents. Considering ingrained beliefs on how our upbringing has shaped us, the desire to believe that we ourselves are wonderful parents, and further, the entrenched and highly political nature of social constructionist theories in the academy and the related surfeit of parenting manuals, it is no wonder that the profound implications of the second law has failed to pierce the seemingly impenetrable wall of group think. It seems that most psychologists are unaware of or uninterested in the three laws of behavioural genetics and the field in general.

Most people are horrified by the idea that “parenting does not matter”; however, this not what is being stated at all, and the attitude reveals just how far popular belief has skewed our understanding of parenting. A child is a dependent whom we love and for whom we have an ethical and sacred duty to provide. Deliberately abusive parenting is obviously problematic, however, whether by nature or nurture or both, the vast majority of parents are impelled to do their best for their children. What is being stated is that there is no sneaky way to raise a faster, smarter, more well-adjusted and all round better child. As children grow up their personalities will only shine more brightly and we should not hope to optimise our offspring any more than we should hope to optimise our spouse or friends. Science has demonstrated what every parent already knows: that children come into the world and quickly move beyond their control. Therefore it is more important for parents to form an authentic relationship with their children, rather than agonize over personal and societal expectations of being transformative parenting dynamos.

The third law is not as politically controversial as the second law, but nonetheless vital in unravelling the mysteries of who we are. As stated, “substantial portion of the variation in complex human behavioural traits is not accounted for by the effects of genes or families” (Turkheimer, 2000, p. 160). This means that within a given culture, unique experiences and events are responsible for shaping who we are, and that these factors account for roughly 50% of the variation of any given trait. Although the shared experiences of siblings do not make them more or less similar, it seems that individual environments play a substantive role in the formation of behavioural traits. For example, most parents instinctively know that their children’s peer groups have a profound influence on their children. It should be no surprise that immigrant children grow up marked by the language, accent and culture of their peers, and not that of their parents. Interestingly, it has proved nigh on impossible to point to typical environmental circumstances as causal factors in specific enduring behavioural traits (2000). After more than a century of social science, we are left with what Turkheimer calls the “gloomy prospect:” that individual environmental factors are too unpredictable and complex to be conducive to causal theories of particular behavioural traits such as authoritarianism or assertiveness.

Careful processing and reflection upon these three laws should not cause the social constructionist any lasting consternation. They do not reify racism, sexism or the status quo because these are fundamentally not genetic issues. To be sure, it is almost certainly true that genetic factors are important in forming prejudiced behaviour, but that is not to say that the problem cannot be solved because genetic factors exist. Indeed, it is likely that our problems can only be solved by paying attention to what is real, and that means we must always consider both nature and nurture as being roughly equally important whenever we consider the aetiology of any behavioural trait. The nature-nurture debate is indeed over. The eugenicists were wrong, and so were the social constructionists. Within a given culture, the unique experiences of an individual are potent factors in shaping behaviour, only matched in importance and magnitude by nature’s blueprint.


Galton, F. (1869). Hereditary genius. London: Macmillan.

Pinker, S. (2002). The blank slate: The modern denial of human nature. New York: Penguin Group.

Turkheimer, E. (2000). Three laws of behavior genetics and what they mean. Current Directions in Psychological Science, 9(5), 160-164.

© 2010, Aaron Michaux