Scarr's original twin study was done in Philadelphia. Seems strange to discount the Scarr-Rowe effect as not existing in the US at all when it was originally found there in the first place.
Scarr’s original study was done over 50 years ago on kids who grew up in pre-Civil Rights America. That level of deprivation is, as I’ve said, virtually unseen in today’s America. Recent studies usually fail to replicate it on more recent experimental data.