For traits, the effect on non-double first cousins can be linearly approximated, which (as far as I know) is a good approximation for inbreeding depression. Fo

For traits, the effect on non-double first cousins can be linearly approximated, which (as far as I know) is a good approximation for inbreeding depression. For example, taking Fluid Intelligence Score (basically how smart you are), in Table 3, its extrapolated effect size is -3.43, so: First cousin (relationship coefficient 0.125, inbreeding coefficient 0.0625) would give -0.214 to FIS. To put that in context, for medication, that would be considered a small effect size. So inbreeding at that level is a bit like taking a stupid pill.