The 'credible source' for that quote is UCLA professor, Jared Diamond, an American evolutionary biologist, physiologist, biogeographer, lecturer and Pulitzer Prize-winning author of 'Guns, Germs, and Steel' (1998),
The information is from his 'In black and white' a chapter in Racism: A Global Reader
By Kevin Reilly, Stephen Kaufman, Angela Bodino

you can read further here pages 23-26 but it is painful and shaming reading.

As to your final question Riginslinger, I have no answer other than - because they could