Francis Galton first coined the phrase nature versus nurture
in the nineteenth century. He also introduced a powerful method for studying this conundrum: statistical analysis of human twins. Identical twins (Figure 1-1
), or monozygotic twins
, share 100% of their genes in almost all cells, as they are products of the same fertilized egg, or zygote
. One can compare specific traits among thousands of pairs of identical twins to see how correlated they are within each pair. For example, if we compare the intelligence quotients (IQs)—an estimate of general intelligence—of any two random people in the population, the correlation is 0. (Correlation is a statistic of resemblance that ranges from 0, indicating no resemblance, to 1, indicating perfect resemblance.) This correlation is 0.86 for identical twins (Figure 1-2
), a striking similarity. However, identical twins also usually grow up in the same environment, so this correlation alone does not help us distinguish between the contributions of genes and the environment.
Fortunately, human populations provide a second group that allows researchers to tease apart the influence of genetic and environmental factors. Nonidentical (fraternal) twins occur more often than identical twins in most human populations. These are called dizygotic twins
because they originate from two independent eggs fertilized by two independent sperm. As full siblings, dizygotic twins are 50% identical in their genes according to Mendel’s laws of inheritance. However, like
monozygotic twins, dizygotic twins usually share very similar prenatal and postnatal environments. Thus, the differences between traits exhibited by monozygotic and dizygotic twins should result from the differences in 50% of their genes. In our example, the correlation of IQ scores between dizygotic twins is 0.60 (Figure 1-2
Behavioral geneticists use the term heritability
to describe the contribution of genetic differences to trait differences. Heritability is defined as the difference between the correlations of monozygotic and dizygotic twins multiplied by 2 (because the genetic difference is 50% between monozygotic and dizygotic twins). Thus, the heritability of IQ is (0.86 − 0.60) × 2 = 0.52. Roughly speaking, then, genetic differences account for about half of the variation
in IQ scores within human populations. Traditionally, the non-nature component has been presumed to come from environmental factors. However, “environmental factors” as calculated in twin studies include all
factors not inherited from the parents’ DNA. These include the postnatal environment, which is what we typically think of as nurture, but also prenatal environment, stochasticity in developmental processes, somatic mutations (alterations in DNA sequences in somatic cells after fertilization), and gene expression changes due to epigenetic modifications
modifications refer to changes made to DNA and chromatin that do not modify DNA sequences but can alter gene expression—these include DNA methylation and various modifications of histones, the protein component of chromatin. As we will learn later, all of these factors contribute to nervous system development, function, and behavior.
Twin studies have been used to estimate the heritability of many human traits, ranging from height (~90%) to the chance of developing
schizophrenia (60–80%). An important caveat regarding these estimates is that most human traits result from complex interactions between genes and the environment, and heritability itself can change with the environment. Still, twin studies offer valuable insights into the relative contributions of genes and nongenetic factors to many aspects of brain function and dysfunction in a given environment. The completion of the
Human Genome Project and the development of tools permitting detailed examination of the genome sequence data, combined with a long history of medical and psychological studies of human subjects, have made our own species the subject of a growing body of
neurobiological research (Section 14.5
). However, mechanistic understanding of how genes and the environment influence brain development, function, and behavior requires experimental manipulations that often can be carried out only in
animal models. The use of vertebrate and invertebrate model species (Sections 14.1–14.4) has yielded much of what we have learned about the brain and behavior. Many principles of neurobiology revealed by experiments on specific model species have turned out to operate in a wide variety of organisms, including humans.