Environmental Factors Influence IQ

There has been a great deal of skepticism expressed that education has any impact on IQ, but a large meta-analysis of research studies suggests that education does significantly increase IQ. 

Intelligence test scores and educational duration are positively correlated. This correlation could be interpreted in two ways: Students with greater propensity for intelligence go on to complete more education, or a longer education increases intelligence. We meta-analyzed three categories of quasiexperimental studies of educational effects on intelligence: those estimating education-intelligence associations after controlling for earlier intelligence, those using compulsory schooling policy changes as instrumental variables, and those using regression-discontinuity designs on school-entry age cutoffs. Across 142 effect sizes from 42 data sets involving over 600,000 participants, we found consistent evidence for beneficial effects of education on cognitive abilities of approximately 1 to 5 IQ points for an additional year of education. Moderator analyses indicated that the effects persisted across the life span and were present on all broad categories of cognitive ability studied. Education appears to be the most consistent, robust, and durable method yet to be identified for raising intelligence.

Stuard J. Ritchie and Elliot M. Tucker-Drob, "How Much Does Education Improve Intelligence? A Meta-Analysis" Psychological Science (June 18, 2018).

This is in line with recent research on the Flynn Effect and Anti-Flynn Effect which shows that changes nationwide in average intelligence are driven by environmental factors.

Significance 

Using administrative register data with information on family relationships and cognitive ability for three decades of Norwegian male birth cohorts, we show that the increase, turning point, and decline of the Flynn effect can be recovered from within-family variation in intelligence scores. This establishes that the large changes in average cohort intelligence reflect environmental factors and not changing composition of parents, which in turn rules out several prominent hypotheses for retrograde Flynn effects. 

Abstract 

Population intelligence quotients increased throughout the 20th century—a phenomenon known as the Flynn effect—although recent years have seen a slowdown or reversal of this trend in several countries. To distinguish between the large set of proposed explanations, we categorize hypothesized causal factors by whether they accommodate the existence of within-family Flynn effects. Using administrative register data and cognitive ability scores from military conscription data covering three decades of Norwegian birth cohorts (1962–1991), we show that the observed Flynn effect, its turning point, and subsequent decline can all be fully recovered from within-family variation. The analysis controls for all factors shared by siblings and finds no evidence for prominent causal hypotheses of the decline implicating genes and environmental factors that vary between, but not within, families.

Bernt Bratsberg and Ole Rogeberg, "Flynn effect and its reversal are both environmentally caused" PNAS (June 11, 2018) https://doi.org/10.1073/pnas.1718793115 (pay per view).

Of course, it has always been uncontroversial that negative environmental factors like pre-natal nutritional deficiencies and lead poisoning in children can lead to lower IQ. 

Probably the most mainstream position is that genetics quite strongly predict maximum intelligence, and that environmental factors, including lack of exposure to the quality education necessary to nurture your potential IQ, can prevent you from reaching your potential.

But since adequate education is so widespread in developed countries where nutrition and other heath factors that reduce IQ are rare for much of the population, our inaccurate intuition from data in these places is that environment plays a relatively minor role in determining IQ. This also leads us to underestimate the genetic IQ potential of populations abroad and domestically that suffer environmental impairments to IQ.