Editing Race and intelligence (section)

===Health and nutrition===
{{Main|Impact of health on intelligence}}
[[File:Lead levels children.png|thumb|Percentage of children aged 1–5 with blood lead levels ''at least'' 10 μg/dL. Black and Hispanic children have much higher levels than white children. A 10 μg/dL increase in blood lead at 24 months is associated with a 5.8-point decline in IQ.<ref name="Bellinger, Stiles & Needleman 1992">{{harvnb|Bellinger|Stiles|Needleman|1992}}</ref> Although the Geometric Mean Blood Lead Levels (GM BLL) are declining, a CDC report (2002) states that: "However, the GM BLL for non-Hispanic black children remains higher than that for Mexican-American and non-Hispanic white children, indicating that differences in risk for exposure still persist."<ref>{{harvnb|MMWR|2005}}</ref>|380x380px]]

Environmental factors including [[Lead poisoning#Effects on children|childhood lead exposure]],<ref name="Bellinger, Stiles & Needleman 1992"/> low rates of [[breast feeding]],<ref name="Campbell et al. 2002">{{harvnb|Campbell et al.|2002}}</ref> and poor [[nutrition]]<ref>{{harvnb|Ivanovic et al.|2004}}</ref><ref>{{harvnb|Saloojee| Pettifor|2001}}</ref> are significantly correlated with poor cognitive development and functioning. For example, childhood exposure to {{nowrap|lead{{tsp}}{{mdash}}{{tsp}}}}associated with homes in poorer {{nowrap|areas<ref>{{Cite web |url=https://www.atsdr.cdc.gov/csem/ped_env_health/docs/ped_env_health.pdf |title=Principles of Pediatric Environmental Health, The Child as Susceptible Host: A Developmental Approach to Pediatric Environmental Medicine |last=Agency For Toxic Substances And Disease Registry Case Studies In Environmental Medicine (CSEM) |date=2012-02-15 |website=U.S. Department for Health and Human Services |access-date=2019-01-30 |archive-date=2019-01-31 |archive-url=https://web.archive.org/web/20190131093309/https://www.atsdr.cdc.gov/csem/ped_env_health/docs/ped_env_health.pdf |url-status=live }}</ref>{{hsp}}{{mdash}}{{tsp}}}}correlates with an average IQ drop of 7 points,<ref name="Lanphear Hornung Khoury Yolton 2005 pp. 894–899">{{cite journal |last1=Lanphear |first1=Bruce P. |last2=Hornung |first2=Richard |last3=Khoury |first3=Jane |last4=Yolton |first4=Kimberly |last5=Baghurst |first5=Peter |last6=Bellinger |first6=David C. |last7=Canfield |first7=Richard L. |last8=Dietrich |first8=Kim N. |last9=Bornschein |first9=Robert |last10=Greene |first10=Tom |last11=Rothenberg |first11=Stephen J. |last12=Needleman |first12=Herbert L. |last13=Schnaas |first13=Lourdes |last14=Wasserman |first14=Gail |last15=Graziano |first15=Joseph |last16=Roberts |first16=Russell |title=Low-Level Environmental Lead Exposure and Children's Intellectual Function: An International Pooled Analysis |journal=Environmental Health Perspectives |volume=113 |issue=7 |date=2005-03-18 |issn=0091-6765 |pmid=16002379 |pmc=1257652 |doi=10.1289/ehp.7688 |pages=894–899|bibcode=2005EnvHP.113..894L }}</ref> and [[Cretinism|iodine deficiency causes a decline]], on average, of 12 IQ points.<ref>{{harvnb|Qian et al.|2005}}</ref><ref>{{cite journal |first1=James |last1=Feyrer |first2=Dimitra |last2=Politi |first3=David N. |last3=Weil |title=The Cognitive Effects of Micronutrient Deficiency: Evidence from Salt Iodization in the United States |year=2017 |journal=Journal of the European Economic Association |volume=15 |issue=2 |pages=355–387 |doi=10.1093/jeea/jvw002 |pmid=31853231 |pmc=6919660 |url=http://www.nber.org/papers/w19233.pdf |access-date=2019-07-22 |archive-date=2020-08-13 |archive-url=https://web.archive.org/web/20200813174601/https://www.nber.org/papers/w19233.pdf |url-status=live }}</ref> Such impairments may sometimes be permanent, but in some cases they be partially or wholly compensated for by later growth. 
  
The first two years of life are critical for malnutrition, the consequences of which are often irreversible and include poor cognitive development, educability, and future economic productivity.<ref>[http://www.thelancet.com/series/maternal-and-child-undernutrition The Lancet Series on Maternal and Child Undernutrition] {{Webarchive|url=https://web.archive.org/web/20110717005704/http://www.thelancet.com/series/maternal-and-child-undernutrition |date=2011-07-17 }}, 2008.</ref><!--Which paper? Link points only to to listed series of papers (yes, need more specific, preferably secondary, reference) --> Mackintosh points out that, for American black people, infant mortality is about twice as high as for white people, and low birth weight is twice as prevalent. At the same time, white mothers are twice as likely to breastfeed their infants, and breastfeeding is directly correlated with IQ for low-birth-weight infants. In this way, a wide number of health-related factors which influence IQ are unequally distributed between the two groups.{{sfn|Mackintosh|2011|pages=343–44}}

The [[Copenhagen consensus]] in 2004 stated that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that one-third of the total global population is affected by [[iodine deficiency]]. In developing countries, it is estimated that 40% of children aged four and under have [[anaemia]] because of insufficient iron in their diets.<ref>{{harvnb|Behrman|Alderman|Hoddinott|2004}}</ref>

Other scholars have found that simply the standard of nutrition has a significant effect on population intelligence, and that the Flynn effect may be caused by increasing nutrition standards across the world.<ref>{{cite journal |last1=Colom |first1=R. |last2=Lluis-Font |first2=J. M. |last3=Andrés-Pueyo |first3=A. |year=2005 |title=The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: supporting evidence for the nutrition hypothesis |journal=Intelligence |volume=33 |pages=83–91 |doi=10.1016/j.intell.2004.07.010}}</ref> James Flynn has himself argued against this view.<ref>{{cite journal |last1=Flynn |first1=J. R. |year=2009a |title=Requiem for nutrition as the cause of IQ gains: Raven's gains in Britain 1938 to 2008 |journal=Economics and Human Biology |volume=7 |issue=1 |pages=18–27 |doi=10.1016/j.ehb.2009.01.009 |pmid=19251490}}</ref>

Some recent research has argued that the retardation caused in brain development by [[infectious disease]]s, many of which are more prevalent in non-white populations, may be an important factor in explaining the differences in IQ between different regions of the world. The findings of this research, showing the correlation between IQ, race and infectious diseases was also shown to apply to the IQ gap in the US, suggesting that this may be an important environmental factor.<ref name="Eppig 2011">{{harvnb|Eppig|2011}}</ref>

A 2013 meta-analysis by the World Health Organization found that, after controlling for maternal IQ, breastfeeding was associated with IQ gains of 2.19 points. The authors suggest that this relationship is causal but state that the practical significance of this gain is debatable; however, they highlight one study suggesting an association between breastfeeding and academic performance in Brazil, where "breastfeeding duration does not present marked variability by socioeconomic position."<ref>{{Cite web |title=Long-term effects of breastfeeding – a systemic review |first1=Bernardo L. |last1=Horta |first2=Cesar G. |last2=Victoria |publisher=World Health Organization |year=2013 |access-date=18 June 2018 |url=http://apps.who.int/iris/bitstream/handle/10665/79198/9789241505307_eng.pdf |archive-date=9 April 2020 |archive-url=https://web.archive.org/web/20200409233115/https://apps.who.int/iris/bitstream/handle/10665/79198/9789241505307_eng.pdf |url-status=live }}</ref> Colen and Ramey (2014) similarly find that controlling for sibling comparisons within families, rather than between families, reduces the correlation between breastfeeding status and WISC IQ scores by nearly a third, but further find the relationship between breastfeeding duration and WISC IQ scores to be insignificant. They suggest that "much of the beneficial long-term effects typically attributed to breastfeeding, per se, may primarily be due to selection pressures into infant feeding practices along key demographic characteristics such as race and socioeconomic status."<ref>{{cite journal |first1=Cynthia G. |last1=Colen |first2=David M. |last2=Ramey |journal=Social Science & Medicine |volume=109 |issue=1 |pages=55–65 |year=2014 |pmc=4077166 |title=Is Breast Truly Best? Estimating the Effect of Breastfeeding on Long-term Child Wellbeing in the United States Using Sibling Comparisons |doi=10.1016/j.socscimed.2014.01.027 |pmid=24698713}}</ref> Reichman estimates that no more than 3 to 4% of the black–white IQ gap can be explained by black–white disparities in low birth weight.<ref>{{harvnb|Reichman|2005}}</ref>