how do we know what inbreeding depression costs? From recent studies of inbred individuals?
Yes, that’s how one would do it. The usual reference is to a study of Japanese cousin-marriages back in the ’50s or so where IIRC the estimate was <5 IQ points, but there’s been research since then, of course; a google for ‘inbreeding depression intelligence’ should bring some research to light.
Being confined to the subscales that look like pattern-matching and analogies, IIRC; I’m not sure which paper I get this from, but it seems Jensen does at least make this claim in http://www.charlesdarwinresearch.org/2010%20Editorial%20for%20Intelligence.pdf and in some citations in http://menghusblog.wordpress.com/2013/06/21/explanation-behind-the-non-g-gains-in-the-flynn-effect-introducing-the-measurement-invariance-model/ so there’s some starting points at least.
Yes, that’s how one would do it. The usual reference is to a study of Japanese cousin-marriages back in the ’50s or so where IIRC the estimate was <5 IQ points, but there’s been research since then, of course; a google for ‘inbreeding depression intelligence’ should bring some research to light.