Editing Birth order (section)

==Intelligence==
{{Further|Dysgenics|Fertility and intelligence}}
[[File:Siblings 1890 hg.jpg|thumb|upright|Three siblings from the 1890s]]
In a metanalysis, Polit and Falbo (1988) found that firstborns, only children, and children with one sibling all score higher on tests of verbal ability than later-borns and children with multiple siblings.<ref>{{cite journal |author1=Polit D. F. |author2=Falbo T. | year = 1988 | title = The intellectual achievement of only children | journal = Journal of Biosocial Science | volume = 20 | issue = 3| pages = 275–285 | pmid = 3063715 | doi=10.1017/S0021932000006611|s2cid=34618696 }}</ref> 

[[Robert Zajonc]] argued for a "confluence" model in which the lack of siblings experienced by firstborns exposes them to the more intellectual adult family environment. This predicts similar increases in IQ for siblings who next-oldest sibling is at least five years senior. These children are considered to be "functional firstborns". The theory further predicts that firstborns will be more intelligent than only children, because the latter will not benefit from the "tutor effect" (i.e. teaching younger siblings).<ref>{{Cite journal |last1=Zajonc |first1=R. B. |last2=Markus |first2=Gregory B. |last3=Berbaum |first3=Michael L. |last4=Bargh |first4=John A. |last5=Moreland |first5=Richard L. |date=1991 |title=One Justified Criticism Plus Three Flawed Analyses Equals Two Unwarranted Conclusions: A Reply to Retherford and Sewell |url=https://www.jstor.org/stable/2095776 |journal=American Sociological Review |volume=56 |issue=2 |pages=159–165 |doi=10.2307/2095776 |jstor=2095776 |issn=0003-1224|url-access=subscription }}</ref>

Several studies have found that firstborns have slightly higher [[IQ]] than later borns.<ref>{{cite journal |author1=Belmont, M. |author2= Marolla, F.A. | title=Birth order, family size, and intelligence | journal=Science | year=1973 | volume=182 | pages=1096&ndash;1101 | doi=10.1126/science.182.4117.1096 | pmid=4750607 | issue=4117|bibcode= 1973Sci...182.1096B |s2cid= 148641822 }}</ref><ref name=":0" /> Such data is, however, commonly confounded with family size,<ref name="Roberts2015" /><ref>Guang Guo; Leah K. VanWey (1999). "Sibship Size and Intellectual Development: Is the Relationship Causal?" ''American Sociological Review'', 64(2), 169-187. doi:000312249906400202</ref> which is in turn correlated with IQ confounds, such as social status. Likewise, an analysis of data from the [[National Child Development Study]] has been used in support of an alternate ''admixture hypothesis'', which asserts that the apparent birth-order effect on intelligence is wholly an artifact of family size,<ref>{{cite journal|author=Satoshi Kanazawa|author-link=Satoshi Kanazawa|title=Intelligence, Birth Order, and Family Size|journal=Personality and Social Psychology Bulletin|volume=38|issue=9|pages=1157–64|year=2012|doi=10.1177/0146167212445911|pmid=22581677|s2cid=14512411}}</ref> i.e. an instance of [[selection pressure]] acing against intelligence under modern conditions.

The claim that firstborns have higher IQ scores to begin with, has, however, also been disputed outright. Data from the [[National Longitudinal Surveys|National Longitudinal Survey of Youth]] show no relationship between birth order and intelligence.<ref name="Rowe2000">{{cite journal | pmid = 10892201 | year = 2000 | last1 = Rodgers | first1 = JL | last2 = Cleveland | first2 = HH | last3 = Van Den Oord | first3 = E | last4 = Rowe | first4 = DC | title = Resolving the debate over birth order, family size, and intelligence | volume = 55 | issue = 6 | pages = 599–612 | journal = The American Psychologist | doi = 10.1037/0003-066X.55.6.599 }}</ref>