"Nature versus nurture" refers to an ongoing debate about how much genes are responsible for an individual's traits, compared to how much is due to the environment around the person.
There are some areas where genetics clearly dominate: certain hereditary diseases, for example, such as cystic fibrosis, or hair and eye color. And what language a child ends up speaking is entirely determined by his or her environment. But other traits appear to develop from a combination of both influences.
For example, a person's height as an adult is determined to some extent by his or her genes, but environmental factors such as diet can also impact height. The same is true for weight. Some women are genetically predisposed to store more body fat in particular areas of the body (around the abdomen versus the hips), but this propensity can be controlled through diet and exercise. The issue becomes even more complex when dealing with the brain. Hormones help build structures in the brain, so genetics clearly play a role in such matters as distinctly male or female behaviors. But the brain is also designed to be flexible, enabling it to adapt to environmental experiences.
In order to better determine genetic and environmental influences, scientists study sets of twins.
Non-identical twins have genes as different as any other sibling, but they share a very similar environment, so genetic differences can be more accurately studied.
Identical twins share the same genes, and often the same environment, but some are separated at a young age and raised in different homes. Since they share all the same genes but experience a very different home environment, the differences between them are likely to arise from environmental factors.
Recent twin studies have shown that shortsightedness is largely genetic, for example; only 15 percent of cases are caused by environmental factors like using computers or reading lots of small print.