Ronald C. Johnson argued that if early environmental stimulation or deprivation has a significant effect on intellectual ability, then individuals who are genetically identical and who are exposed to a common early environment should resemble one another more closely in IQ than similar individuals who have not shared a common environment. Johnson compared the IQ's of 23 pairs of twins separated at different times. He found that twins who were separated after they were 1 year old resembled one another significantly less closely than early separated twins. A Danish study of 12 pairs of Juel-Nielsen (1962) supported this result. A study by Shields in 1962 found no significant difference in the scores of 48 pairs of twins on (1) Raven's Dominoes Intelligence Test and (2) the synonyms part of the Mill Hill Vocabulary Scale, regardless of the age of the pair of separation. (WD)