Left-handed (N = 109) and right-handed (N = 115) undergraduates (99 males, 125 females) received the SIBT (a "mental rotation" test), the 3DD (3-dimensional drawing test), and a family sinistrality (FS) questionnaire. Left-handers were further separated into consistent left-handed (CLH) and inconsistent left-handed (ILH) subgroups, based on consistency of hand preference. On the spatial tests, males outperformed females, with no overall handedness effects. Also, for males, CLH's (but not ILH's) performed significantly worse than right-handers on the SIBT, but this difference was not found on the 3DD. For females, no handedness subgroup differences were found on either spatial tasks. Familial sinistrality was twice as common in left-handers as in right-handers. Among males, the incidence of FS+ in the CLH subgroup also was over twice that for ILH's. Thus, where left-handers report a greater incidence of FS+, and are inferior to right-handers in mental rotation skill, it is CLH's (particularly males) who contribute mostly to these effects. The results suggest that previous discrepant findings reported in studies of the cognitive correlates of left-handedness may be due in part to the mixing of two distinct left-handed subgroups. An understanding of the differences between them may be crucial for better understanding the genetic and neurobiological mechanisms underlying handedness.