The handedness theory of Yeo and Gangestad (1993) posits that moderate right-handedness is the modal manifestation of genetic handedness and that ''developmental instability'' produces deviations from modal handedness. It is also suggested (1) that sinistral parents may produce fewer offspring than do dextral parents; and (2) that sinistral mothers may be more prone to miscarriages than are dextral mothers. In line with these hypotheses, Gangestad et al. (1996) reported that a human leukocyte antigen (B8) was related to both left-handedness and to reduced offspring number in their study. They also found that left-handedness was related to the human leukocyte antigen DR3, and Yeo and Gangestad (1998) noted that this antigen has been found by Christiansen et al. (1996) to be associated with an increased risk of spontaneous abortion in women. We assessed the first hypothesis through a study of the family sizes of 2083 families with two right-handed parents and 502 families having one or more left-handed parents; we assessed the second hypothesis from miscarriage history data supplied by 300 dextral and 52 sinistral mothers. Results supported the developmental instability theory with respect to the hypothesis regarding family size, but not with respect to the hypothesis regarding miscarriage-proneness.