Unexpected and powerful evidence of the importance of relatedness has come from sex ratio studies. As noted above, haplodiploid female workers are related to full sisters by 3/4 and to brothers by 1/4. This implies that, other things being equal, these workers ought to prefer helping sisters (Hamilton, 1972). Specifically, Trivers and Hare (1976) showed that in a colony with a single once-mated queen, workers should prefer to invest three times as much in future queens as in males. They also showed that, in species of ants likely to have a single, once-mated queen, investment ratios are in fact closer to this 3:1 ratio than to the usual Fisherian 1:1.

More impressive evidence comes from species with variable relatedness. When a queen mates multiple times, workers will not favor this 3:1 ratio because the workers are equally related (by 1/4) to half sisters and brothers (because brothers do not have fathers, multiple mating by the queen does not change their relatedness). This means that these workers should rear more males than those in singly mated colonies, and the frequency-dependent nature of sex ratio selection should cause the two kinds of colonies to increasingly specialize (Boomsma and Grafen, 1990, 1991). Workers in colonies with singly mated queens should specialize largely in rearing queens, and workers in colonies with multiply mated queens should specialize in rearing males. This odd prediction of what has come to be called “split sex ratio theory” was strikingly confirmed in a study of the ant Formica exsecta (Sundström et al., 1996) and has been confirmed in many other species (Queller and Strassmann, 1998; Chapuisat and Keller, 1999; Bourke, 2005).

Besides showing that workers are indeed sensitive to relatedness, the sex ratio studies made an even more important point: There can be conflict within these apparently superorganismal colonies. Queens are equally related to their sons and daughters, so they should prefer the standard 1:1 sex investment ratio (Trivers and Hare, 1976). The resulting conflict can lead to inefficiencies that are decidedly against the interests of the superorganism as a whole. For example, the split sex ratio described above for Formica exsecta is achieved only after some waste. Queens in both singly and multiply mated colonies laid the same sex ratio of eggs, but workers in the singly mated colonies achieved their preferred investment in full sisters by killing many of the male larvae (Sundström et al., 1996; Chapuisat et al., 1997).

It seems paradoxical that this elegant evidence for kin selection theory comes from conflict rather than from cooperation, but there is really no contradiction. Kin selection theory shows how individuals can further the reproduction of their own genes, and this is sometimes achieved by cooperation and sometimes by conflict.