Researchers of child psychiatry examine the performance of children in several environments. Some are better than others in spite of the same gene. Most of the degree in cognitive function is genetically determined, but there are some rooms to be improved by several ways of training.
It is often said that children longitudinally exposed to an abuse and domestic violence are likely to become impaired in cognitive function. They mark inferior to other boys and girls. Some researchers claim the possibility that physical and psychological abuse can bring the onset of bipolar disorder.
As well, children in a divorced family are often deemed to be in a tough environment. But, the evidence is scarce to prove that divorce of the parents makes their children dull.
Then, how are the children of the same sex parents? A recent research has denied the difference in some domains of their lives. The researchers gathered several studies for three decades. They concluded that children of the same sex couple were as well as other children emotionally, socially, and educationally, as a result of the investigation of a total of 79 studies.
The Guardian: Children raised by same-sex parents do as well as their peers, study shows
It is a hopeful finding. But it is possible that there is a selection bias to draw this conclusion. Even if the research protocol is valid and fair, editors of most academic journals seem not to accept an article claiming children of a same-sex couple is inferior to others because this idea is potentially discriminating.
I believe children of a same-sex parent perform equally as other children if they are grown up in a proper environment. But it is not easy to prove this simple issue.