monozygotic

And, yes, there are monozygotic twins who are discordant for IQ, so there must be influences other than genes on IQ.

Given both these effects it is amazing how similar many monozygotic twins are to each other.

Also with monozygotic twins the pre birth environment is not identical either.

We explored the genetic influence on focal GM and WM densities in magnetic resonance brain images of 54 monozygotic and 58 dizygotic twin pairs and 34 of their siblings.

The possibility of 2 non-monozygotic twins having identical genes is not zero, just astronomically low.

Adult monozygotic twins are equally similar in physiology and psychological traits.

I'd bet that the heritability of slavery in the 19th century was actually pretty low - I wouldn't imagine that monozygotic twin pairs are any more/less likely to be be slaves than same-sex dizygotic counterparts.

For example, my two American nieces who are monozygotic twins were brought up in the same household by the same parents, but they do not look indistinguishably "identical," but rather can be told apart readily by their parents and other close relatives and told apart with careful thought by other people who know them.

Most human behavioral characteristics are polygenic, that is they are influenced by many genes, each with a small effect, and even monozygotic twins are consistently found to differ in highly heritable characteristics, for environmental reasons that are sometimes still quite poorly understood.