How Genes Influence Behavior is a book about behavioral genetics by three scientists:
Dr. Jonathan Flint: professor of neuroscience at Oxford
Dr. Ralph Greenspan: Associate Director of the Kavli Institute for Brain and Mind at UCSD
Dr. Kenneth Kendler: professor of psychiatry at Virginia Commonwealth School of Medicine
This post has some notes from the book.
• At least ten family studies of schizophrenia have shown that close relatives of individuals with schizophrenia have approximately ten times the risk of suffering from the illness themselves.
• How to determine the difference between genetic and shared environmental factors:
1. If the correlation in identical twins is at least twice as large as in fraternal twins, twin resemblance is likely to be due entirely to genetic factors.
2. If the correlations are approximately the same magnitude in both types of twin, twin resemblance is likely to be due to shared environmental effects.
3. If the magnitude of the correlation in fraternal twins is between 50 and 100% of that seen in identical twins, then it is likely that both genetic and shared environmental effects are operating.
• Heritability is the proportion of variation in a phenotype that is due to the genetic differences between individuals in the population.
• Over ten studies have estimated the heritability of liability to schizophrenia to be 81%.
• Studies have found estimates of the heritability of alcoholism in the range of 50 to 60%.
• A meta-analysis of twin studies shows the heritability of major depression as 37%.
• A review of studies found that the heritability for all five major dimensions of personality were in the range of 30-45%.
• The estimated genetic correlation between generalized anxiety disorder and major depression is +1.00. From a genetic perspective, the two syndromes are for all intents and purposes the same disorder.
• A study of adoptees in Iowa found that in those who had a low genetic risk, adding a high-risk environment produced a minimal increase in antisocial behavior. However, in those who had a high level of genetic vulnerability, adding a high-risk environment produced a very large increase in antisocial behaviors.
• A series of twin studies has shown that social support has a heritability of over 50%.
• A longitudinal study of male twins in Virginia showed that as twins got older, the genetic influences on peer deviance got stronger and stronger.
• A study of Finnish patients with delayed sleep phase syndrome found that the long allele of the hPer3 gene favored morning preference and the short allele favored evening preference.
• Four criteria are useful for judging the validity of a claim that “X is a gene for Y”:
1. the strength of the association
2. the specificity of the relationship
3. the non-contingency of the effect
4. the causal proximity of X to Y
• Heritability varies between psychiatric disorders with relatively consistent results across studies:
20 to 30% for most anxiety disorders
30 to 40% for major depression
50 to 60% for alcoholism
80% or higher for schizophrenia, bipolar illness, and autism