Proactive and reactive personality traits have been documented in many species and refer to collections of broadly assigned characteristics. Proactive individuals are described as being bold and active explorers. Reactive individuals are wary of novelty and cautious of change. Since there is evidence that these personality types are susceptible to different stress-related health problems, it would be of value to have a means of detecting each type easily. In primates, hand preference might provide such a measure since it is associated with personality type: right-handed primates approach, touch and explore novel objects, environments and foods more readily than left-handed primates. We have investigated social behaviour associated with hand preference in common marmosets. Left- and right-handed marmosets were presented with live crickets, an unfamiliar food item, both when alone and with their social companion. In right-handed marmosets the latency to begin capturing crickets was shorter when the companion was present than when the marmoset was tested alone: latency 67±23 seconds (mean and standard error) with companion present and 410±141s when alone (Wilcoxon, Z=-2.43, P=0.02). Left-handed marmosets were unaffected by the presence of a companion: latency 289±124s when with companion and 476±190s when alone (Wilcoxon, Z=-0.73, P=0.46). Right-handed marmosets also performed more mobbing responses directed at a model predator than did left-handed marmosets, which is another example of a relationship between hand preference and social behaviour (e.g. mean number of 'tsik' mobbing calls by right- and left-handed marmosets were 185±23 and 119±19, respectively; Man Whitney U-Test, 2-Tailed, U=133.5, P=0.03). These findings may be useful in providing the best captive care for the two personality types.