So there were no differences between these boys and girls in ability, nor in past history of success.
The key word is these. As in, “These bright boys and bright girls both had roughly the same ability and history of success, having all performed in the top of their class in every subject.”
The key word is these. As in, “These bright boys and bright girls both had roughly the same ability and history of success, having all performed in the top of their class in every subject.”
I stumbled across that interpretation here. It seems reasonable but sloppy.