Have you collected data on family occupational backgrounds?

Speculation: There's some dynamic where the children of professionals are drawn to the same occupational area as their parents, but don't want to follow the same path directly. I'd expect the children of engineers to be particularly likely to express an interest in physics.

EDIT because I can't resist: Many of my friends who were the children of lawyers studied philosophy, explicitly disavowed law...and then went to law school.