We suggest that one distal cue that natural selection has prepared our minds to use when making predictions about social environments is the intrauterin flow of nutrients from mother to the self as indexed by the birth weight of the self. Given the importance of the mother's social environment for the birth weight of the child, birth weight provides an indirect but hard-to-fake cue to the larger social environment in which the child will come to be situated in. Because negative social factors (i.e., lack of social support) predict low birth weight, we predict that low birth weight is utilized as a cue of harsher social environments and give rise to avoidant social strategies. We test this hypothesis in three studies. Consistent with our prediction - and controlling for potential confounds - we find that low birth weight increases social distrust and social avoidance.