They found some papers where children with autism or ADHD often were found walking a way that causes an “anterior pelvic tilt”, which makes your butt stick out more. I myself personally happen to have such a tilt while being autistic and yeah, my butt sticks out more. 
Fun fact: in Dutch ASD is “autismespectrumstoornis” aka “ASS”
Bit idea: nazi who’s way into phrenology but he’s gotten it mixed up with proctology
Interested to know how that would work. I don’t think the anus is big enough to have the wide variety of measurements you get with phrenology. Maybe it would just be a closeted guy claiming that his superior elasticity and accommodation for girth and length are proof of his superior genetics, and that male members of lesser (i.e. black) races desire female ass due to their underdeveloped abilities to receive anal penetration.
Someone tried to imagine the concept of superior Aryan assholes, and I followed that logic to its natural conclusion.