Many twin studies have failed to find a strong connection between the home environment and personality.

Psychogenic theories in general have become increasingly unpopular, particularly since twin studies have shown that autism is highly heritable.

Although twin studies suggest that ASDs are highly heritable, specific genes have been difficult to identify.

However, the utility and accuracy of these twin studies has been called into question and remains controversial.

Another source of data indicating a significant biological aspect of some mental health disorders is twin studies.

A 1975 twin study found that identical twins were no more likely than fraternal twins to both have the same phenotype for tongue rolling.

As with many twin studies, the numbers of individuals studied is relatively limited.

Differences in human height is 60%-80% heritable, according to several twin studies and has been considered polygenic since the Mendelian-biometrician debate a hundred years ago.

The structure of language abilities at 4 years: A twin study.

Aggression has been examined via both twin studies and adoption studies.