Nature vs. Nurture
It is obviously known in the psychology world that both heredity and environment shape how our behavior matures. However, the extent to which affects you more is not known (and will probably never be fully known), and scientists are constantly conducting experiments to learn more about nature versus nurture. To find more about hereditability, scientists use behavioral genetic designs, which include family studies, twin studies, and adoption studies. In family studies, the researcher studies how a trait runs in an intact family, meaning the family members have been raised in the same house under the same conditions. Twin studies are done (obviously) amongst identical twins. Because the twins are more similar genetically than non-twin siblings, a twin study allows the researcher to find out how the twins' genes affect their behavior. Adoption studies are done between adopted children who are not like their parents genetically but allow the researcher to see how their environment affects the adopted child (Lilenfeld 116-117). By conducting these studies, the researchers can see how much genes and environment affect the behavior of the families.
On an article on the PBS Nova website, some scientists came to the theory that the nature vs. nurture issue swings heavily to the nurture side. The researchers did a study on what had been recently discovered as a "cancer gene". On a group of 45,000 sets of twins, the researchers concluded that it is ones environment that causes cancer and not-so-much hereditability. They conducted a twin study to find out the correlation between the twins' environments and if they get cancer. Although this study had profound affects on the cancer gene theory, it does not disprove the theory of the cancer gene.