One problem is that separated identical twins do not actually provide a perfect nature-versus-nurture template.

ECONOMIST: WHAT WE LEARN FROM TWINS: The mirror of your soul | The

In the nature-versus-nurture debate, psychologists say fathers play an important role in the men their sons become.

CNN: STORY HIGHLIGHTS

As soon as he knew a visitor's political view, said Fisher, he also could predict where he would stand on the nature-versus-nurture issue in genetics.

ECONOMIST: Famous economists