Dr. Nancy Siegel

One of the boys started to exhibit demonic possession.

Well, I've always been interested in psychology.

And so that as a major was a natural for me.

But I'm also a fraternal twin.

I have a sister who looks and acts nothing like me.

And that was the draw for twin studies.

Well, as far as the science goes, twins are special because it's a very simple and yet very elegant methodology for looking at the genetic and environmental influences underlying behavior.

What we do is we compare the similarities of genetically identical twins to those of genetically non-identical or fraternal twins.

And if the identical twins are more alike in running speed, in intelligence, in personality, then we know that the genes play some role in the development of that trait.

I can tell you a lot about the Jim twins.

The Jim twins were in the state of Ohio.

They were born about 40, 50 miles apart in different cities.

That's where they grew up.

And then when they met at the age of 39, they discovered a whole long list of similarities in addition to their first names.

They both had mixed headache syndromes when they were teenagers.

They both put on the same amount of weight at a certain point in adulthood.

They both liked to do woodworking, and they both had worked part-time in sheriff's offices.

They both worked part-time in McDonald's.

They both bit their fingernails to the nub.

They both married Linda, divorced Linda and married Betty.