The increasingly influential longtermism philosophy advocates prioritizing resource allocations to relatively larger future populations to maximize utilitarian gains. Yet, emerging research seeking to promote long-term intergenerational concern has overlooked a key impediment which likely constrains intergenerational attitudes: human moral psychology. We investigate moral judgments and social consequences of altruistic tradeoffs in generational closeness for gains in welfare, finding the ethical principles of longtermism are misaligned with folk moral intuitions. Four US Prolific samples (N=2,306) reveal that altruists who prioritize future welfare over generational closeness are perceived as less moral and face social repercussions in both attitudes and behavior. Nonetheless, strong intergenerational concern can mitigate these negative judgments. As society grapples with near-term and long-term challenges, these insights help reconcile prevailing debates in psychological, philosophical, and public discourse, emphasizing key features decision-makers and policymakers should consider when weighing the immediate and future needs of society.