Diverse strands of evidence suggest that schizophrenia is associated with an excess of left and mixed handedness, reflecting anomalous cerebral lateralization. Genetic studies have indicated a degree of overlap between bipolar disorder (BPD) and schizophrenia. Nevertheless, pattern of handedness and degree of lateralization have not been explicitly tested in BPD. We measured handedness, footedness and relative manual dexterity in a sample of 47 families comprising BPD probands and their bipolar‐spectrum and unaffected relatives (N = 240). The BPD I sample (N = 55) was significantly more lateralized on handedness, footedness and relative manual dexterity than their unaffected relatives (N = 66). They were also more lateralized than their relatives with other psychiatric diagnoses. No evidence of excess mixed handedness or footedness was observed in the BPD I sample. We raise the possibility that schizophrenia and BPD I differ in that disproportionate left‐hemisphere dominance in BPD I is associated with right‐hemisphere dysfunction leading to deficits in emotional regulation. Given our results, we hypothesized that degree of lateralization may be a phenotypic marker or endophenotype for BPD I. We therefore conducted a family‐based genetic association analysis with this quantitative trait. Relative hand skill was significantly associated with a functional variant in the catechol‐O‐methyltransferase gene. We speculate that this polymorphism may influence brain lateralization.