Chronic parental maltreatment has been associated with lower levels of interpersonal trust, and depriving environments have been shown to predict short‐sighted, risk‐averse decision‐making. The present study examined whether a circumscribed period of adverse care occurring only early in life was associated with biases in trust behavior. Fifty‐three post‐institutionalized (PI) youth, adopted internationally on average by 1 year of age, and 33 never‐institutionalized, non‐adopted youth (Mage = 12.9 years) played a trust game. Participants decided whether or not to share coins with a different anonymous peer in each trial with the potential to receive a larger number of coins in return. Trials were presented in blocks that varied in the degree to which the peers behaved in a trustworthy (reciprocal) or untrustworthy (non‐reciprocal) manner. A comparison condition consisted of a computerized lottery with the same choices and probabilistic risk as the peer trials. Non‐adopted comparison youth showed a tendency to share more with peers than to invest in the lottery and tended to maintain their level of sharing across trials despite experiencing trials in which peers failed to reciprocate. In contrast, PI children, particularly those who were adopted over 1 year of age, shared less with peers than they invested in the lottery and quickly adapted their sharing behavior to peers’ responses. These results suggest that PI youth were more mistrusting, more sensitive to both defection and reciprocation, and potentially more accurate in their trusting decisions than comparison youth. Results support the presence of a sensitive period for the development of trust in others, whereby conditions early in life may set long‐term biases in decision‐making. Using a developmentally appropriate trust game, trust behavior was examined in post‐institutionalized (PI) adolescents internationally adopted as infants or young children. As shown here, in contrast with non‐adopted comparison youth, PI youth, particularly those who were adopted over a year of age, were more mistrusting of peers at baseline (i.e., no feedback) and more sensitive to both defection (i.e., low reciprocation) and high reciprocation. Findings suggest that conditions early in life may set long‐term biases in decision‐making.