Editing Flynn effect (section)

===Nutrition===
{{See also|Iodine deficiency#Deficient populations}}
Improved nutrition is another possible explanation. Today's average adult from an industrialized nation is taller than a comparable adult of a century ago. That increase of stature, likely the result of general improvements in nutrition and health, has been at a rate of more than a centimeter per decade. Available data suggest that these gains have been accompanied by analogous increases in head size, and by an increase in the average size of the brain.<ref name="Neisser97"/><ref>{{cite journal |last1=Jantz |first1=R. |last2=Meadows Jantz |first2=L. |year=2000 |title=Secular change in craniofacial morphology |journal=American Journal of Human Biology |volume=12 |issue= 3|pages=327–38 |doi=10.1002/(SICI)1520-6300(200005/06)12:3<327::AID-AJHB3>3.0.CO;2-1 |pmid=11534023|s2cid=22059721 |doi-access=free }}</ref> This argument had been thought to suffer the difficulty that groups who tend to be of smaller overall body size (e.g. women, or people of Asian ancestry) do not have lower average IQs.<ref name=TO1987/>

A 2005 study presented data supporting the nutrition hypothesis, which predicts that gains will occur predominantly at the low end of the IQ distribution, where nutritional deprivation is probably most severe.<ref name="Colom2005"/> An alternative interpretation of [[skewed]] IQ gains could be that improved education has been particularly important for this group.<ref name=TO1987/>

A century ago, [[micronutrient|nutritional]] deficiencies may have limited body and organ functionality, including skull volume. The first two years of life are a critical time for nutrition. The consequences of malnutrition can be irreversible and may include poor cognitive development, educability, and future economic productivity.<ref>{{cite web |title=The Lancet Series on Maternal and Child Undernutrition |year=2008 |url=http://www.thelancet.com/series/maternal-and-child-undernutrition |access-date=February 11, 2011 |archive-date=July 17, 2011 |archive-url=https://web.archive.org/web/20110717005704/http://www.thelancet.com/series/maternal-and-child-undernutrition |url-status=live }}</ref> On the other hand, Flynn has pointed to 20-point gains on Dutch military ([[Raven's Progressive Matrices|Raven's]] type) IQ tests between 1952, 1962, 1972, and 1982. In 1962 he observed that Dutch 18-year-olds had a major nutritional handicap. They were either in the womb or were recently born, during the great [[Dutch famine of 1944]]—when German troops monopolized food and 18,000 people died of starvation.<ref>C. Banning (1946). "Food Shortage and Public Health, First Half of 1945". ''Annals of the American Academy of Political and Social Science'' Vol. 245, The Netherlands during German Occupation (May 1946), pp. 93–110</ref> Yet, concludes Flynn, "they do not show up even as a blip in the pattern of Dutch IQ gains. It is as if the famine had never occurred."<ref name="PB101-171">{{cite journal | author = Flynn J.R. | year = 1987 | title = Massive IQ gains in 14 nations: What IQ tests really measure | journal = Psychological Bulletin | volume = 101 | issue = 2| pages = 171–91 |doi=10.1037/0033-2909.101.2.171}}</ref><ref>Flynn, James R. (2009). ''What Is Intelligence?'' (p. 103). Cambridge University Press. Kindle Edition.</ref> It appears that the effects of diet are gradual, taking effect over decades (affecting mother as well as the child) rather than a few months.{{Citation needed|reason=Reliable source needed for the whole sentence|date=February 2015}}

In support of the nutritional hypothesis, it is known that, in the United States, the average height before 1900 was about 10&nbsp;cm (~4&nbsp;inches) shorter than it is today.<ref>{{cite journal |last1=Samaras |first1=Thomas T. |last2=Elrick |first2=Harold |date=May 2002 |title=Group Height, body size, and longevity: is smaller better for the human body? |journal=West J Med |volume=176 |issue=3 |pages=206–08 |pmc=1071721 |pmid=12016250 |doi=10.1136/ewjm.176.3.206}}</ref> Possibly related to the Flynn effect is a similar change of [[cranial vault|skull]] size and shape during the last 150 years. A Norwegian study found that height gains were strongly correlated with intelligence gains until the cessation of height gains in military conscript cohorts towards the end of the 1980s.<ref name="doi10.1016/j.intell.2004.06.004">{{Cite journal | doi = 10.1016/j.intell.2004.06.004| last1 = Sundet | first1 = J. | last2 = Barlaug | first2 = D. | last3 = Torjussen | first3 = T. | journal = Intelligence | volume = 32 | issue = 4 | pages = 349–62 |title=The end of the Flynn effect?: A study of secular trends in mean intelligence test scores of Norwegian conscripts during half a century| year = 2004 }}</ref> Both height and skull size increases probably result from a combination of [[phenotypic plasticity]] and genetic [[Selection (biology)|selection]] over this period.<ref>{{cite journal |vauthors=Jantz RL, Meadows Jantz L |title=Secular change in craniofacial morphology |journal=Am. J. Hum. Biol. |volume=12 |issue=3 |pages=327–38 |date=May 2000 |pmid=11534023 |doi=10.1002/(SICI)1520-6300(200005/06)12:3<327::AID-AJHB3>3.0.CO;2-1|s2cid=22059721 |doi-access=free }}<br />
{{cite journal |author=Jantz RL |title=Cranial change in Americans: 1850–1975 |journal=J. Forensic Sci. |volume=46 |issue=4 |pages=784–87 |date=July 2001 |doi=10.1520/JFS15047J |pmid=11451056 }}</ref> With only five or six human generations in 150 years, time for [[natural selection]] has been very limited, suggesting that increased skeletal size resulting from changes in population [[phenotype]]s is more likely than recent genetic evolution.

It is well known that [[micronutrient]] deficiencies change the development of intelligence. For instance, one study has found that [[iodine deficiency]] causes a fall, on average, of 12 IQ points in China.<ref>{{Cite journal|author=Qian M |title=The effects of iodine on intelligence in children: a meta-analysis of studies conducted in China |journal=Asia Pacific Journal of Clinical Nutrition |volume=14 |issue=1 |pages=32–42 |year=2005 |pmid=15734706|author2=Wang D|author3=Watkins WE|display-authors=3|last4=Gebski|first4=V|last5=Yan|first5=YQ|last6=Li|first6=M|last7=Chen|first7=ZP}}</ref>

Scientists James Feyrer, Dimitra Politi, and David N. Weil have found in the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas. Journalist Max Nisen has stated that with this type of salt becoming popular, that "the aggregate effect has been extremely positive."<ref>{{Cite web|last=Nisen|first=Max|title=How Adding Iodine To Salt Resulted In A Decade's Worth Of IQ Gains For The United States|url=https://www.businessinsider.com/iodization-effect-on-iq-2013-7|date=2013-07-22|access-date=2023-01-23|website=Business Insider|language=en-US}}</ref>

Daley et al. (2003) found a significant Flynn effect among children in rural [[Kenya]], and concluded that nutrition was one of the hypothesized explanations that best explained their results (the others were parental literacy and family structure).<ref>{{cite journal|last1=Daley|first1=TC|last2=Whaley|first2=SE|last3=Sigman|first3=MD|last4=Espinosa|first4=MP|last5=Neumann|first5=C|s2cid=12315212|title=IQ on the rise: the Flynn effect in rural Kenyan children.|journal=Psychological Science|date=May 2003|volume=14|issue=3|pages=215–19|pmid=12741743|doi=10.1111/1467-9280.02434}}</ref>