their forms or personality but can never change them entirely. Darwin's Theory of Evolution steered naturalists such as George Williams and William Hamilton to the concept of personality evolution. They suggested that physical organs and also personality is a product of natural selection. With the advent of genomic sequencing, it has become possible to search for and identify specific gene polymorphisms that affect traits such as IQ and personality. These techniques work by tracking the association of differences in a trait of interest with differences in specific molecular markers or functional variants. An example of a visible human trait for which the precise genetic basis of differences are relatively well known is eye color. In contrast to views developed in 1960s that gender identity is primarily learned (which led to a protocol of surgical sex changes in male infants with injured or malformed genitals, such as David Reimer), genomics has provided solid evidence that both sex and gender identities are primarily influenced by genes: Linkage and association studies In their attempts to locate the genes responsible for configuring certain phenotypes, researches resort to two different techniques. Linkage study facilitates the process of determining a specific location in which a gene of interest is located. This methodology is applied only among individuals that are related and does not serve to pinpoint specific genes. It does, however, narrow down the area of search, making it easier to locate one or several genes in the genome which constitute a specific trait. Association studies, on the other hand, are more hypothetic and seek to verify whether a particular genetic variable really influences the phenotype of interest. In association studies it is more common to use case-control approach, comparing the subject with relatively higher or lower hereditary determinants with the control subject. See also References Further reading Articles Books External links Meta-analysis of twin correlations and heritability 20th-century controversies Human behavior Human development Personality theories Race and intelligence controversy Biology controversies Dilemmas Human genetics Psychology controversies Evolutionary psychology