Comment by thaumasiotes

You know, you can just look these things up. This isn't some piece of obscure trivia.

> Which defects?

Ashkenazi Jews are prone to a whole host of genetic defects that affect the brain, of which the most famous is Tay-Sachs.

> What reasons?

A lifestyle focused on commerce as opposed to foraging or agriculture.

> Who is assuming?

Everyone, basically.

In the case of the Ashkenazi Jews, as far as we can tell they've been like that for as long as records of the group have existed. But the French Canadian specialization in commerce postdates the discovery of North America.