What does the research say about the effect of environment on iq scores in poor homes versus affluent homes? what does this suggest?

Research shows that kids with higher socioeconomic status (SES) outperform those with lower SES.  Research shows that the average IQ of children from poor cities is just 80% of their counterparts from rich areas. As their parents income declines so do their kid's reading and memory abilities. This also applies with language abilities.

Stressful enviroment seems to affect a child's development functions such as thinking, learning, focus, develop strong vocabulary and succeed in school.  This suggests that when surviving income is not guaranteed the kids tend to distract from their learning process and slowly become less competent than kids with higher SES, whose only focus is to engage in their studies.