L.P.
Ethnic heritage is also a consideration. Slavics, natives in North and South America and many orientals, tend to have very flat heads. I'm half Slavic, half Spanish. My siblings have perfectly rounded heads but I have quite a flat one, inherited from my father's side of the family.
A previous poster commented on vision problems. I will look into that, since I started using glasses at 6 and have a very high prescription and vision issues (which none of my siblings have). I never knew that there could be a connection there! But then again, my Dad, asides of having a flat head and being borderline genius, didn't need glasses until his 70s.
BTW, they say I'm the smartest of the 4 siblings, flat head or not!