Evolutionary psychologists propose that humans evolved a first line of defense against pathogens: the behavioral immune system (BIS). The BIS is thought to be functionally flexible such that the likelihood and magnitude of BIS activation depends on the individual's perceived vulnerability to disease (PVD). Because conspecifics are sources of infection, the BIS has implications for affiliation. By priming and measuring chronic levels of PVD, we examined PVD's relation to affiliation in zero-acquaintance situations in the laboratory, online, and during speed-dating events. Elevated BIS activation was associated with decreased attraction and affiliative behavior in situations that varied in the trade-off between social reward and potential risk of infection. These results were not due to attachment style, personality traits, or disgust sensitivity. This suggests that in social interactions, approach motivation associated with the need to belong may be weighed against avoidance motivation associated with the need to protect the self from disease.

Infants born with medical problems are at risk for less optimal developmental outcomes. This may be, in part, because neonatal medical problems are associated with maternal distress, which may adversely impact infants. However, the reserve capacity model suggests that an individual's bank of psychosocial resources buffers the adverse effects of later-encountered stressors. This prospective longitudinal study examined whether preexisting maternal psychosocial resources, conceptualized as felt security in close relationships, moderate the association between neonatal medical problems and infant fussing and crying 12 months postpartum. Maternal felt security was measured by assessing its indicators in 5,092 pregnant women. At birth, infants were classified as healthy or having a medical problem. At 12 months, experience sampling was used to assess daily maternal reports of fussing and crying in 135 mothers of infants who were healthy or had medical problems at birth. Confirmatory factor analyses revealed that attachment, relationship quality, self-esteem, and social support can be conceptualized as indicators of a single felt security factor. Multiple regression analyses revealed that prenatal maternal felt security interacts with infant health at birth to predict fussing and crying at 12 months. Among infants born with medical problems, higher felt security predicted decreased fussing and crying. Maternal felt security assessed before birth dampens the association between neonatal medical problems and subsequent infant behavior. This supports the hypothesis that psychosocial resources in reserve can be called upon in the face of a stressor to reduce its adverse effects on the self or others.