I have my doubts that this was a legit study. If you get on a bus and note school-age kids in Germany...the girls are wearing all of the trendy designer clothing (jackets, tennis shoes, etc). The boys are much less so.
That was the obvious answer, pretty all across the western world.
Boys own a few sets of clothes. Girls have a wardrobe.
And that’s fine even though it tends to benefit the girls overall.