SUMMARYBirth weight data on 72078 liveborn singletons representing 5981 paternal and 7036 maternal single first cousin kinships were assembled by computerized record linkage from all marriages and livebirths registered in Hawaii during a 38‐year period from 1942 to 1979. These kinships represented incrosses of five selected racial groups. An additional 15536 livebirths from 1134 paternal and 1679 maternal cousin kinships provided data on outcrossing between racial groups, and 28952 livebirths from 2379 paternal and 3538 maternal cousin kinships provided data on both incrossing and outcrossing within kinships. Phenotypic resemblance in birth weight between single first cousins was estimated by the covariance and intraclass correlation. The covariances between maternal cousins were generally much higher than those between paternal cousins. Neither heterogeneity in maternal genotypes nor heterogeneity in newborn genotypes had any significant effect on the resemblance between first cousins. These observations have led to the conclusion that maternal influences of non‐hereditary origin are of primary importance in determining birth weight in man. The observed non‐zero correlation in several groups of paternal cousins was explained in part by the process of preferential selection of mates within and among racial groups by brothers, and probably by the similarity in some maternal characteristics associated with age at childbearing among their