Recruitment is usually negatively density-dependent with fewer offspring surviving when more are produced. Parental care could alter the pattern as behaviours that maximize individual fitness are not necessarily adaptive at the population level. We manipulated the number of eggs spawned into the nests of male threespine stickleback, and found egg survival to be positively density-dependent. This reversed negative density-dependent survival observed in the absence of parental care. The reversal was caused my males investing more in parental care when receiving more eggs, while favouring future reproductive opportunities when receiving few eggs. Density-dependent parental care thus amplified changes in offspring production in relation to number of eggs spawned. Such amplification may occur in disturbed environments where human activities have altered female fecundity and males may receive more or less eggs than expected. The optimal balancing between present and future parental investment can then be distorted, resulting in maladaptive parental behaviour that reduces offspring survival. These results suggest that behaviours that have evolved to maximize individual fitness under pristine conditions can become mal-adaptive under disturbed conditions and influence the recruitment of offspring into a population. Considering that human activities are rapidly transforming environments, such mal-adaptive behavioural responses could be common and magnify negative effects of human activities on population dynamics.