A review of the twin literature and analyses of two large twin samples found identical twin correlations higher than fraternal twin correlations by about .20 for a variety of traits of ability, personality and interests. This was interpreted as indicating that about half of the variation among people in a broad spectrum of psychological traits is due to differences among the people in genetic characteristics. The data also suggest that the environmental influences on ability affect twins reared together in the same way, while the environmental influences on personality and interests affect twins in the same family differently. The implications of these findings for psychological theory, for the nature-nurture controversy, for the explanation of the recent national decline in aptitude test scores, and for future human evolution are discussed. (Author/MS)