 In 2009, it was proposed that the larger brain size conferred by a high number of Olduvai domain copies in humans carried an evolutionary advantage which led to the persistence and maintenance of Olduvai copies within this high range.At the same time, the Olduvai domains, like many other repetitive genetic elements, are highly susceptible to increases and decreases in number of copies, through duplications or deletions, and the researchers referenced various studies from 2005 to 2009 that found that a higher number of copies contributed to autism severity while a lower number contributed to schizophrenia severity.Since these disorders are fairly common among humans, it was proposed that this explained their prevalence.