This paper exploits a unique ongoing experiment to analyze the effects of early rearing conditions on physical and mental health in a sample of rhesus monkeys (Macaca mulatta). We analyze the health records of 231 monkeys that were randomly allocated at birth across three rearing conditions: mother rearing, peer rearing, and surrogate peer rearing. We show that the lack of a secure attachment relationship in the early years engendered by adverse rearing conditions has detrimental long-term effects on health that are not compensated for by a normal social environment later in life.