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http://www.pnas.org/content/early/2018/06/05/1718793115.short

Flynn effect and its reversal are both environmentally caused
Bernt Bratsberg and Ole Rogeberg
PNAS published ahead of print June 11, 2018 https://doi.org/10.1073/pnas.1718793115
Edited by Richard E. Nisbett, University of Michigan, Ann Arbor, MI, and approved May 14, 2018 (received for review October 27, 2017)

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.

Footnotes
-B.B. and O.R. contributed equally to this work.
-To whom correspondence should be addressed. Email: ole.rogeberg@frisch.uio.no.
-Author contributions: B.B. and O.R. designed research, performed research, analyzed data, and wrote the paper.
-The authors declare no conflict of interest.
-This article is a PNAS Direct Submission.
-This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1718793115/-/DCSupplemental.

https://en.wikipedia.org/wiki/Flynn_effect#Possible_end_of_progression

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