Editing Flynn effect (section)

===Schooling and test familiarity===
The duration of average schooling has increased steadily. However, a criticism of this explanation is that if (in the United States) older and younger subjects, with similar educational levels, are compared together, then the IQ gains appear almost undiminished in each group compared to when they are considered individually.<ref name="Neisser97"/>

Many studies find that children who do not attend school score drastically lower on the tests than their regularly attending peers. During the 1960s, when some [[Virginia]] counties [[Massive resistance|closed their public schools to avoid racial integration]], compensatory private schooling was available only for White children. On average, the scores of African-American children who received no formal education during that period decreased at a rate of about six IQ points per year.<ref name="Neisser97"/>

Another explanation is an increased familiarity of the general population with tests and testing. For example, children who take the very same IQ test a second time usually gain five or six points. However, this seems to set an upper limit on the effects of test sophistication. One problem with this explanation and others related to schooling is that in the US, the groups with greater test familiarity show smaller IQ increases.<ref name="Neisser97"/>

[[Early intervention]] programs have shown mixed results. Some preschool (ages 3–4) intervention programs like "[[Head Start Program|Head Start]]" do not produce lasting changes of IQ, although they may confer other benefits.{{which|date=July 2014}} The "[[Abecedarian Early Intervention Project]]", an all-day program that provided various forms of [[Environmental enrichment (neural)|environmental enrichment]] to children from infancy onward, showed IQ gains that did not diminish over time. The IQ gains in the experimental group compared to the control group was 4.4 points. These gains persisted until at least age 21.<ref>{{cite book |author1=Plomin R. |author1-link=Robert Plomin|author2=DeFries J.C. |author3=Craig I.W. |author4=McGuffin P. |title=Behavioral genetics in the postgenomic era |year=2003 |edition=4th }}</ref>

Citing a high correlation between rising literacy rates and gains in IQ, [[David Marks (psychologist)|David Marks]] has argued that the Flynn effect is caused by changes in literacy rates.<ref>{{Cite journal |last=Marks |first=David Francis |date=2010-06-01 |title=IQ Variations across Time, Race, and Nationality: An Artifact of Differences in Literacy Skills |url=https://journals.sagepub.com/doi/10.2466/pr0.106.3.643-664 |journal=Psychological Reports |language=en |volume=106 |issue=3 |pages=643–664 |doi=10.2466/pr0.106.3.643-664 |pmid=20712152 |issn=0033-2941 |via=[[Sage Publishing]]}}</ref>