> A study (1999) by Capron and Duyme of French children adopted between the ages of four and six examined the influence of socioeconomic status (SES). The children's IQs initially averaged 77, putting them near retardation. Most were abused or neglected as infants, then shunted from one foster home or institution to the next. Nine years later after adoption, when they were on average 14 years old, they retook the IQ tests, and all of them did better. The amount they improved was directly related to the adopting family's socioeconomic status. "Children adopted by farmers and laborers had average IQ scores of 85.5; those placed with middle-class families had average scores of 92. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."
This demonstrates quite clearly that high socioeconomic status causes high IQ scores, not the other way around.
Your other source is the Pumpkin Person who's blog is titled "The psychology of horror" and writes about horror movies. Does not appear credible.
That results indicates that if you take children and go from "abused or neglected as infants, then shunted from one foster home or institution to the next" to a stable adopted family home, you can increase IQ scores.
Which is pretty obvious. There is always an amount of abuse that will destroy someone's IQ. Bringing them back to a decent middle-class upbringing will restore them to their baseline.
But once that baseline is achieved, evidence is that it can't generally be improved. One can learn skills and knowledge but IQ is stable. Above lower-middle class American abuse-free households, increases in SES don't affect IQ.
E.g. People have inherent talent at sprinting. Malnutrition or abuse can destroy this talent. But once people have decent upbringing, and try hard, and have support, what determines the champion is genetics. Which is why pretty much all champion sprinters have west African ancestry (since they have inherited traits that make them good sprinters).
EDIT: Here's a source for you on IQ heritability: American Psychological Association’s Task Force on Intelligence report.
“If one simply combines all available correlations in a single analysis, the heritability (h2) works out to about .50 and the between-family variance (c2) to about .25.” And “By late adolescence h2 is around .75 and c2 is quite low (zero in some studies).” (p. 85).
In the case of the foster children, they initially scored low on IQ test but once placed in a better environment their scores went up significantly. This demonstrates that environment can have a huge effect on IQ scores and casts doubt on tests which claim that a correlation between IQ and family members implies IQ is mostly a result of genetics. Since family members generally live in similar environments their similar IQ scores can be explained by environment. It is even plausible that IQ score is mostly a result of environment.
Now, I can not prove that to be the case but it is far from settled that better genes = better IQ = better socioeconomic status. That reasoning implies that people in third world countries are poor because they are stupid which I just don't believe, partly because I've actually been to some of those countries.
Can you back up these claims? I don't think either is obviously true. IQ tests themselves have dubious value.