According to Martin Schiller, Ph.D., executive director of the Nevada Institute of Personalized Medicine at the University of Nevada, Las Vegas, whether you're talking about a trait, behavior or disease, it's partially inherited and partially the result of your environment. So how do scientists figure out what is inherited and what's influenced by external factors?

Schiller said geneticists estimate the breakdown by doing pedigree and twin heritability studies. As it sounds, these studies are conducted on identical or fraternal twins. They aim to work out how much of a trait, behavior or disease comes down to:

  • Heritability (genetics)
  • Shared environment (things that happen to both twins and affect them equally)
  • Non-shared environment (things that happen to one twin and not the other or affect each twin differently)

Nathan A. Gillespie, Ph.D., is an