Between the lines: If identical twins are about as similar to each other as fraternal twins are to each other on some trait, that suggests the environment is doing a lot of the work.

• If identical twins are more similar to each other than fraternal twins, that points to a stronger genetic influence.

What they're saying: Twin research has found that "almost everything is influenced by genes to some extent," says Sylia Wilson, associate professor at the Minnesota Center for Twin and Family Research.

• That includes "personality, psychiatric conditions, likes, dislikes," she says.
• Decades ago, there was much more emphasis on family environment. That often meant "the default was to blame the family, and in particular the mother" when people were struggling, Wilson says.

Yes, but: Because twins start out with so much in common, researchers can also use their differences to see how individual experiences may affect the body.

What they found: In a twin study, Wilson and colleagues found that when twins differed in their alcohol use, the heavier-drinking twin tended to have a smaller hippocampus, a brain region involved with memory.

• "So that does suggest that there's some effect of alcohol on that particular region of the brain," she says.

Zoom in: In 2020, Wilson started an infant twin registry in Minnesota, continuing the work of her predecessors who kept records of identical and fraternal twins in the area.

• The registry invites families to participate in studies on behavior and relationships.

The bottom line: Twin research isn't just about twins. It's a tool for understanding everyone.