Twin studies often yield fascinating results – and sometimes not the results we expect. For example, several twin studies over the years have shown that identical twins share the same similarities whether raised together or apart; identical twins also tend to become more similar psychologically as they age, versus their fraternal counterparts.

Here are some interesting findings from past and recent twin studies, regarding heritability and environmental influences on twins.

Spouse Selection is Not Genetic

Contrary to common belief, identical twins do not tend to marry spouses who are similar to one another. The Minnesota Twin Family study found that the spouses of identical twin sets are no more alike than random pairings of people, and that 66% of identical twins actively disliked their twin's spouse.

The Tendency to Divorce is Genetic

The same study found that if an identical twin's twin divorced, the chances of the other twin divorcing rose to 45%. In fraternal twin pairs, the rate only increased to 30%, leading researchers to conclude that genetics plays at least a small role in the tendency to divorce. (more info available at Minnesota Twin Family Study https://www.psych.umn.edu/psylabs/mtfs/)

ADD, Behavior Problems, and Drug Use is Genetically-based

The Mid-Atlantic Twin Registry found that identical twins had a higher rate of ADD, behavior problems, and drug use if the other twin engaged in these behaviors. However, the study also found that these behaviors were much less likely in people who valued religion in their lives.

Obesity and Insomnia are Genetically-based

A University of Washington Twin Study found that identical twins are more likely to suffer from obesity and/or insomnia if their co-twin is affected.

Language Development is Mostly Environmental

In a study done by the Wisconsin Twin Project, researchers found that toddler use of expressive language was mostly influenced by environment; however, boys showed a higher level of genetic influence on vocabulary, and girls showed a higher level of genetic influence on combining words.

Making Smart, Nice Friends is Not Genetic

A Michigan State University twin study found that the tendency of children with high intelligence and good social skills to have friends with the same characteristics (or not) was not directly influenced by genetics; instead, environment seemed to be a more important factor.

Dyslexia is Genetically-based

An Australian study sponsored by Queensland Institute of Medical Research has found a strong genetic correlation in spelling and dyslexia, and isolated genetic regions involved with the disorder. (more info available at https://genepi.qimr.edu.au/general/researchtopics.html)