Ever since Charles Darwin, scientists studying evolution have grappled with how cooperative behaviours that are costly to an individual but beneficial to others can be consistent with maximal fitness. In 1981, writing in Science, Robert Axelrod and William D. Hamilton1 presented key findings on how such cooperation can evolve. They accomplished this by weaving together two emerging ideas of the time in this area of research. Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any time Subscribe to this journal Receive 51 print issues and online access $199.00 per year only $3.90 per issue Rent or buy this article Prices vary by article type from$1.95 to$39.95 Prices may be subject to local taxes which are calculated during checkout Nature 651, 894-895 (2026) doi: https://doi.org/10.1038/d41586-026-00802-7 Axelrod, R. & Hamilton, W. D. Science 211, 1390–1396 (1981). Article PubMed Google Scholar Maynard Smith, J. & Price, G. R.…