What is heritability? The heritability of a trait in a given population tells us what proportion of differences in that trait is due to genetic differences. It provides an answer to the main question in the nature–nurture controversy.

Heritability can be calculated both for physical and psychological characteristics but it generates controversy mainly in the context of human behavioral traits like intelligence, personality differences, criminality, etc. For this reason I will focus here only on discussions of heritability in psychology (i.e., human behavior genetics). Criticisms of heritability claims in this area are frequently based on a curious mixture of methodological objections and warnings about political motives and/or implications of this research. A systematic study that would try to disentangle all the argumentative threads that are often run together in this contentious debate was long overdue. So this book was simply waiting to be written.

I was introduced to the nature–nurture debate by reading Ned Block and Gerald Dworkin's well-known and widely cited anthology about the IQ controversy (Block & Dworkin 1976a). This collection of articles has long been the main source of information about the heredity–environment problem for a great number of scientists, philosophers, and other academics. It is not an exaggeration to say that the book has been the major influence on thinking about this question for many years.