<p>Altus’ review of the literature (Science, 1966) on the relation of birth order to eminence, intelligence, college attendance and other social parameters was generally consistent in finding first-borns to be over-represented. Complementary evidence at the lower end of the intellectual continuum following an extensive review of the men­tal deficiency literature appeared non-existent. To partially fill this lacuna, an effort was made to provide evidence concerning the rela­tion between ordinal position of birth and mental deficiency. Data were obtained for 4,398 institutionalized retardates. First-borns with siblings were compared with later borns on the variables of family size, sex, ethnicity, intelligence, and socioeconomic status, yielding significant differences between observed and expected fre­quency distributions on groupings of family size, intelligence, and father’s educational level. Only-children were studied separately; the distribution being significantly different from expectation on the variables of ethnicity, intelligence, and father’s educational attain­ment. Two methodological issues relevant to birth order studies are considered and implications are discussed in terms of family interaction patterns.</p>