ercent. However, attempts to quantify heritability have serious problems in explaining some of the data. Interestingly, heritability estimates vary with age: A study of 60 year old (average age) Swedish twins (some reared together and others apart) indicates that heritability increases from about 40 percent in childhood to about 60 percent in early adulthood and about 80 percent in later life (Plomin, 1994, Plomin & Petrill, 1997). Eyesenck argues that this is because "we structure our environment based on genetic drives (p. 42)." He reasons that environment exerts a greater influence on children, who have little choice; as they age, diversity and availability of choices expands, "and if these choices are at least partially determined by genetic factors, the influence of environment is thereby diminished." Benno (1990) suggests the difficulty in determining relative contributions lies in the interdependency of genes and environment.Lewontin, Rose, and Kamen (1984) suggest that heritability of IQ is irrelevant and unimportant, as heritability is not synonymous with unchangeability. They attribute this confusion to a general misunderstanding about genes and development. They assert that the genotype is inherited and unchanging; the phenotype is in a constant state of flux, involving morphological, physiological, and behavioral properties. In simpler terms, the loss of a limb is irreversible, but not heritable; Wilson's disease is heritable but not irreversible. They may be mistaken about the heritability of genotype: Stoltenberg and Hirsch (in press) explain that parental genotypes may not be passed down because they are broken up at meiosis and a new genotype is formed at conception.One of the consequences of the Human Genome Project, tasked with sequencing the entire human complement of DNA, is a public perception that scientists are developing a molecular understanding of the human condition. Seldom a month goes by without a media arti...
