twin studies

It seems to me that you are quick to dismiss twin studies because their designs are not perfect, despite the fact that they all seem to "fail" in the same direction.

Also despite the fact that they comport with what most of us have observed in our personal lives.

As an example of the latter - I don't want to go into details here for obvious reasons, but I was once part of a group which included a young man who was, to put it in the colloquial term, quite a bit of a spazz. He got married and invited everyone to his wedding. I was unable to go but the others told me afterwards that they met his extended family and they were all just like him, except for one young woman who was perfectly normal and intelligent. They found this odd until it was revealed to them that she had been adopted.

Most people have stories like this. We have all seen it, even if it is not always so clear cut. So why go to such extremes to deny what everyone has seen? It seems very strange, and smacks of a desire to deceive.