Figure 1. Competing effects of birth selection on hypothesis testing.

Upper panel shows separate distributions of fitness, H, for unwanted children on the left and wanted children on the right. A is mean fitness for unwanted children and B is mean fitness for wanted children. Hypothesis is that B>A Middle panel illustrates positive selection. Left truncation from biological culling prior to birth would shift mean fitness of observable unwanted children towards the right making it more difficult to test that B> A^C. Lower panel illustrates adverse selection. Right truncation from family planning would shift mean fitness of observable unwanted children towards the left increasing the chance of concluding that B> A^FP.