Cultural evolutionary models may also shed light on why although evidence of behavioral modernity exists before 50,000 years ago it is not expressed consistently until that point. With small population sizes, human groups would have been affected by demographic and cultural evolutionary forces that may not have allowed for complex cultural traits. According to some authors until population density became significantly high, complex traits could not have been maintained effectively. Some genetic evidence supports a dramatic increase in population size before human migration out of Africa. High local extinction rates within a population also can significantly decrease the amount of diversity in neutral cultural traits, regardless of cognitive ability.