Friday, October 10, 2014

Heritability estimates: do they equal degree of genetic causation?

This is the first entry in a series on causation, mental problems and criminality that will run on the blog for the next month or so as Forensic Fridays. This first entry will deal with genetics and what can be inferred from heritability estimates. The series will start broadly with causation in neuroscience and behavioural sciences and later focus on forensic psychiatry and the use of mental health expertise in courts of law, where sometimes diverging understandings of causation come to the fore.

Almost every type of behaviour, personality trait or liability for mental health problem that has been subjected to family studies (esp twin studies), has been shown to be to some extent under genetic influence. The proportion of the population variance ascribable to genetic effects is usually between 40 and 80%, with the higher end of the range reserved for narrow phenotypes such as autism, schizophrenia or bipolar disorder. Personality traits typically come in around 50%, while some behaviour patterns are higher, like persistent aggressive antisocial behaviours at 65% (Burt and coworkers, 2009). The great value of this research is that it has proved beyond any reasonable doubt that life is unjust (as if we hadn't guessed before, but the blank-slate model was actually commonly defended not that long ago ). Humans indeed come into the world differently equipped and have different possibilities of abstaining from substance abuse, violence and other destructive behaviours. Let this inspire some humility in all of us.

But then things get less clear-cut. Very few gene variants that account for more than a percent of the variance have been identified, and, even when combined, such effects amount to far less than the heritability calculated in family studies. This is referred to as the “missing heritability”. But why has it been so hard to identify the responsible gene variants? There is a high degree of complexity in the integrations between genes (meaning that each carries a small effect size on the group level but may exert large effects in specific families or individuals), there are epigenetic modifications, gene x environment interactions and many other possible explanations.

But did a heritability of, say, 65%, ever mean that 65% of the causation behind the phenomenon under study was genetic? No, it didn’t, it just said that 65% of the difference in liability across a given population at a  given time could statistically be referred to genetic effects. Consider this: over history, sociocultural changes have had very large effects on, for example, violence. The introduction of a strong state in the 15-17th centuries reduced violence by factors of 10 to 100. During such a time frame, very little genetic change happened. Yet the spectrum of violence in society changed dramatically.

Or take the example of height. If nutrition is lacking, people will be of short stature. Then, genetic effects will be smaller, as the general growth is hampered by an environmental factor and the overall variance is constrained. But when nutrition is adequate, the role of genes in the total variance will be larger. Children of tall parents will be much taller than children of shorter parents. But wouldn’t this also mean that nutrition is the main cause of someone attaining his/her adult height? 

So, for this week, heritability means that the variation of something in a population is under genetic influence - but it does not measure the degree of genetic causation for the phenomenon per se. Therefore, the missing heritability may not be such an enigma after all. There was never any reason to believe that it would be easy to identify the genes behind non-Mendelian, partly heritable behaviours or health problems. 

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