Neuroscientist and evolutionary psychologist Roger Keynes, psychologist Nicholas Humphrey and medical scientist John Skoyles have argued that the gait of these individuals is due to two rare phenomena coming together, not atavism.[9] First, instead of initially crawling as infants on their knees, they started off learning to move around with a "bear crawl" on their feet.[9] Second, due to their congenital brain impairment, they found balancing on two legs difficult.[9] Because of this, their motor development was channeled into turning their bear crawl into a substitute for bipedalism.[9]