SELECTION AND RELATED THREATS TO GROUP COMPARISONS - AN EXAMPLE COMPARING FACTORIAL STRUCTURES OF HIGHER AND LOWER ABILITY GROUPS OF ADULT TWINS

Is it possible to construct valid explanations of behavioral phenomena from differences found among naturally existing groups? Many of psych ology's cherished findings in such domains as intellectual performance and temperament rest on differences of this kind. A venerable and ric h literature on selection and selection effects both reminds us that n aturally existing groups might or might not derive from a common paren t population and warns that valid decisions between these alternatives are often jeopardized by the very methods on which those decisions re st. We examine these matters within the context of intellectual perfor mance, illustrate key points with an analysis of data, and examine the implications for current research on differences in ability levels.