We have 3 boys (aged 6, 4 and 2) and my BIL and SIL have 2 girls, aged 6 and 2. When we get together, I am always struck by how little "risk" they allow the girls to take.
For example, last time they visited, it was a cold-ish day. My sons asked to go out and play in the garden and I said yes, as long as they put coats on. My SIL hadn't heard me say this and her eldest daughter asked the same, and her response was "No, absolutely not!".
She then relented when she realised our kids were going out, but wrapped her daughter in far more layers than just a coat. We opened the doors, and the boys just tumbled out (without us) as they always do - we have a big garden but can see it all very well from the kitchen, where we were all sat. But SIL insisted BIL went out with niece and stayed close to her at all times. Youngest daughter was allowed out eventually, but not allowed to walk across lawn by herself in case she wobbled over.
Our 2 year old was climbing up and down trampoline ladder by himself, as he always does (trampoline has safety net), but niece of same age not allowed to climb things.
Obviously not a scientific study, as only one family, but it did show me how in this family at least, the girls are being given a clear message that they are not as capable of physical tasks as boys. I find it really disappointing.
I don't dispute that there may be generally some gender differences, but (a) they are just generalisations if so, and (b) unfortunately gender socialisation often has a huge influence, whether people realise it or not.