This is a double helix. Its elegant structure is formed from DNA, the genetic code that contains instructions for all living things. But how much of the way we think and behave is stored in this sequence of genetic information? And how much do we receive from the environment around us?

Most of our traits are influenced by many genes. How tall you are, for example, reflects the size of your face, vertebrae, leg bones and so forth, each of which may be influenced by different genes interacting with your environment. Complex traits such as intelligence, happiness, and aggressiveness are similarly influenced by groups of genes. Thus, our genetic predispositions, our genetically influenced traits, help explain both our shared human nature and our human diversity.

Heredity is what we get when our parents shuffle their gene decks and deliver a hand to us. And it turns out in many, many ways to be very important. So our temperament, our personality, our intelligence, are all powerfully influenced by our genes.

It is the pursuit of behavioral genetics to tease apart what is genetically programmed from what is influenced by the environment around us and how the two interact. One of the classic techniques to scientifically tease apart the influences of environment and heredity is to study identical and fraternal twins.

Twin studies are a valuable tool of behavior geneticists. And what they enable us to do is compare genetic clones, identical twins, to see how similar they are compared to fraternal twins, who also are born at the same time and raised in the family. And it turns out that identical twins are more alike in so many different ways than are fraternal twins.

Studies of thousands of twin pairs in the USA, Sweden, Finland, and Australia provide a consistent answer. Measuring traits such as extroversion and neuroticism, identical twins are much more similar than fraternal twins. In a seminal experiment beginning in 1981 by Thomas Bouchard at the University of Minnesota, researchers located and studied 80 pairs of identical twins reared apart. Participants were given tests to measure personality traits, intelligence and abilities, occupational and leisure interests, and social attitudes.

Despite their different rearing, they're often strikingly alike. And that's a powerful illustration of the power of genetic influences.

Another way to study behavior genetics is through adoption studies.

Studies of adopted children, comparing them to their biological parents with whom they share genes, and their adoptive parents with whom they share a nurturing environment, also reveal the power of genes.

The idea is to see does this offspring resemble the biological parents or resemble the adoptive parents?

Twin studies and adoptive family studies do seem to throw the focus on genetics. But to what extent is our behavior genetic? To try to figure this out, behavioral geneticists can mathematically estimate the heritability of a trait. Heritability refers to the extent to which differences among people are influenced by genes.

In an example looking at the heritability of intelligence, the slope of this graph shows the correlation between the average intelligence of parents and their offspring. If we have calculated that the heritability of intelligence in this case is 50%, this does not mean that your intelligence is 50% genetic. Rather it means that genetic influence explains 50% of the observed variation among people.

To take a concrete example, I think it the clearest way to try to explain it. If we wanted to take a look at height and see to what extent do genes and the environment influence individual differences among height.

If the heritability of height is 90%, this does not mean that a 60 inch tall woman can credit her genes entirely for 54 inches and her environment for the other six inches. Whether it is height, personality, or intelligence, we can never say what percentage of an individual's traits are accounted for by their genes. Heritability refers to the extent to which differences among people are influenced by genes.

Another related kind of phenomenon is what's called the gene environment interaction. And that is cases in which our genes determine how sensitive we are to environmental influences.

Today, behavioral geneticists have progressed beyond asking if genes influence behavior. Fundamentally, we know they do. So the question is, what are the specific genes that influence behavior? And how can we identify them in populations?

The molecular approach to behavioral genetics addresses this question from the bottom up and has the power to reveal at risk populations for some of the most prevalent diseases today. In labs worldwide, geneticists team up with psychologists to pinpoint genes that put people at risk for such genetically influenced disorders as learning disabilities, depression, schizophrenia, and alcohol dependence. It is the study of chromosomes using the powerful tools of DNA scanning and genome sequencing. The most powerful potential for DNA sequencing in this capacity is to predict risk so that steps can be taken to prevent problems before they happen.

The molecular approach also opens up the door to a realm of ethical dilemmas. As we begin to identify more specifically the genes that influence a range of faculties, could future parents have the opportunity to screen a developing fetus for these genetic traces? Should we enable parents to screen their fertilized eggs for health and for brains or beauty?

Bipolar disorder, which we know of all the mental disorders, bipolar disorder is probably the most genetic psychiatric disorder. And if you could wave a magic wand and eliminate the risk genes for bipolar disorder, would you do that? Well, bipolar disorder for example, is associated in many interesting ways with creativity. So many of our greatest works of art might not exist if these genetic variants that were associated for risk of bipolar disorder didn't exist in the gene pool.

The pursuits of molecular behavioral genetics have the potential to relieve a great deal of human suffering and to give us insights into the delicate relationship between nature and nurture. But we must keep an open dialogue about the ethical implications of what is possible.