Fig. 1.

The BTOW model. The dominant concedes a fraction P of the group output to the subordinate; the subordinate concedes a fraction Q of the group output to the dominant. There is a tug-of-war in the zone of contested reproductive shares (1 − P − Q) in which the subordinate sacrifices a fraction y of the group output, and the dominant sacrifices a fraction x of the group output, to increase their reproductive shares. The group output is thus reduced by an amount (the shaded region) equal to the sum of these selfish investments.