Recruitment is usually negatively density-dependent in relation to number of offspring produced. However, parental care can alter the pattern, as behaviours that maximise individual fitness are not necessarily beneficial at the population level. We show that in the threespine stickleback, the quality of male parental care is positively density-dependent in relation to number of eggs within a nest, which reverses negative density-dependent egg survival. This is because males invest more in larger broods, while favouring future reproductive opportunities when broods are small. Thus, positively density-dependent parental investment amplifies changes in offspring production when males receive more or less eggs than expected. Notable, parental care investment is mal-adaptive at the individual level when the number of eggs received decreases. Given that female fecundity is changing in many environments because of human disturbances, the results indicate that parental care can contribute to alter population dynamics. At a broader level, the study exemplifies how behaviours that have evolved to maximise individual fitness under pristine condition can become mal-adaptive under disturbed conditions and influence both individual fitness and population processes. Considering that human activities are rapidly transforming environments, such mal-adaptive behavioural responses could be common and magnify negative effects of human activities on populations.