nature-nurture controversy Scientists over the relative importance of environment (upbringing, experience, and learning) and heredity (genetic inheritance) in determining the make-up of an organism, especially as related to human personality and intelligence. Particularly controversial is the reason for differences between individuals, for example in performing intelligence tests. The environmentalist position assumes that individuals do not differ significantly in their inherited mental abilities and that subsequent differences are due to learning, or to differences in early experiences. Opponents insist that certain differences in the capacities of individuals (and hence their behaviour) can be attributed to inherited differences in their genetic make-up.