What we talk about when we talk about heritability

There’s some behavioral evolution stuff I want to talk about soon.  This is great, because it’s what I studied in college and I will be able to contribute something beyond wikipedia synthesis.  In order to really dig into this, I have to explain how we measure heritability.

Heritability“, or h^2, is a very specific measurement in biology.  It measures how much  variation of a particular trait in a particular population is due to genetic variation, relative to the total variation.  It is not a synonym for “genetically determined.”  For example, having two arms is barely heritable, because there’s almost no variation in the number of arms people have.  Of people that have less than two arms, most of them lost them to environmental accidents, not genetics.  There’s no gene for “one arm”, although developmental accidents may have some genetic basis.  Biological sex isn’t very heritable either.  A person’s parents’ sex has very little predictive power on the person’s own sex.    Despite this, number of arms and biological sex is very, very genetically determined

Additionally, a heritability measurement is only valid for the population it was measured in.  In a population in which every individual has exact the same environment will demonstrate much higher heritability for every trait than a population with a varied environment.

Now here is the really weird thing: how sensitive a trait expression is to environmental variation can be influenced by genetics.  High Sensitivity was originally discovered in people who were having severely maladaptive responses to normal stimuli, and was assumed to have a uniform negative effect on people who carried it.  Newer research indicates that the effect is more complicated:  Highly Sensitive People with really good environments do better that Low Sensitive People in those environments.  HSPs in bad environments do worse that LSPs.  Whatever genes cause the trait we call High Sensitivity make a person more sensitive to their environment.  Ellis et al call this biological sensitivity to context.

Lee et al  investigated a particular HSP- associated allele, DRD2 Taq1A, and found that mothers with the gene were harsher parents during a recession than mothers without the gene.  Moreover, they found worsening economic conditions led to worse parenting in everyone, but the effect was more pronounced for mothers with the T allele as opposed to the CC allele.  So a child’s outcome is being affected by their own genetically-driven response to their parents, their parents’ genetically-driven response to the environment, and the actual environment.

I thought what depressed me here was that highly sensitive parents are more likely to have highly sensitive children, which will magnify the negative effects of the recession on children.  Then I read a study showing that maternal psychological trauma was associated with higher disorganization in infants, but only if they had a high-SPS associated allele.*

High Sensitivity is going to come up a lot, so remember that.  But also remember that when journalists talk about heritability, they do not mean what they think they mean.

*The same mutation, DRD4 7-repeat polymorphism, is associated with ADHD.