Although the multiple interacting genes affecting complex traits can readily be dissected, how much genotype–environment interactions contribute to variation in complex traits remains elusive. A recent study that quantified several behavioral phenotypes on the same mouse strains in different laboratories decades apart, shows that some behavioral differences between laboratories remain greatly replicable, whereas others are less robust over time. This report, together with studies from Drosophila, stresses the importance of understanding genotype–environment interactions.