Evolution changes species through time by means of natural selection. But breeds of pets, plants, and livestock have been rapidly changed by human intervention for at least centuries; humans breed animals and plants for a purpose: beauty, docility, meat production, etc. Such breeding techniques are called artificial selection. Inbreeding is really a form of artificial selection, although the traits selected for do not need to be specified. Instead, the population is being bred to be identical, a trend opposite of that observed in nature.

Twin Studies

Twin studies are extremely important in studying human subjects, because humans cannot be bred for certain traits and are hard to manipulate in an experimental fashion. Monozygotic twins, commonly known as identical twins, are genetically identical. Dizygotic twins, or fraternal twins, are no more genetically similar than non-twin siblings are. Therefore, if monozygotic twins share a behavioral trait significantly more frequently than dizygotic twins do, it can be assumed that behavior has a genetic component. It is important to study the shared traits between monozygotic and dizygotic twins, as well as nontwin siblings, because each set of siblings can yield information in comparison with the others. Monozygotic twins and dizygotic twins differ in the amount of genetic material shared, but should have the same prenatal environment, and so behavioral comparisons can isolate genetic components for assessment. Dizygotic twins are no more genetically similar than nontwin siblings, but non-twin siblings do not share their prenatal environment, and so comparison can yield opportunities to asses environmental components.

Knockout Studies

An important technique in molecular analyses of behavioral genetics is the knockout study. Mice are literally designed to express or fail to express certain traits by inserting or subtracting genes from embryonic cells and then reinserting them into a female to gestate. In most knockout studies, a mechanism is designed so that researchers can turn on and off the gene, usually by treatment with an antibiotic. This is accomplished by combining the inserted or deleted gene with another gene susceptible to antibiotics.