Mental and motor abilities and personality development were studied in 20 pairs of infant twins of the same sex on a monthly basis in their first year, that is, before mutual imitation becomes a factor. Blood group determinations, made after the study was completed, revealed an N(1) of 11 fraternal and N(2) of 9 identical pairs. Within-pair differences were significantly greater within fraternal pairs on all tests and rating scales.
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Freedman et al. (1963) studied this question.
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