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Birth order refers to a person's ordinal ranking among siblings — firstborn, second born, and so on. In pop psychology and the self-help movement, birth order is often said to have a profound, deterministic influence on personality, akin to the influence of the stars in astrology.
Some studies have found a correlation between birth order and IQ, but this is subject to debate, and the effect described is very small — 3 IQ points lower for the second born, and 1 point lower still for the third. (The standard deviation on most IQ tests is around 15 points.) Among brothers, birth order is known to have a significant correlation with homosexuality, with every additional older brother increasing the incidence by 33%. 
Frank Sulloway of UC Berkeley, who argues that birth order's effects on personality are significant, has found that openness to new ideas is strongly correlated with a higher order. Later-borns, he argues, accepted the theories of evolution and heliocentrism much more rapidly than firstborns, as well as discredited ideas like phrenology. These findings have not been without controversy.
While popular claims about birth order and personality often devolve into pseudoscience, the topic is also the subject of ongoing legitimate research.
- "The Power of Birth Order", by Jeffrey Kluger; cover story from the 17 October 2007 issue of Time.
- Kristensen, Petter and Tor Bjerkedal. (22 June 2007). Explaining the Relation Between Birth Order and Intelligence. Science 316 (5832): 1717. DOI: 10.1126/science.1141493
- Blanchard, Ray. (September 2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior 40 (2):105-14. PMID: 11534970
- Sulloway, Frank. (2001). Birth Order, Sibling Competition, and Human Behavior. In Harmon R. Holcomb, (Ed.), Conceptual Challenges in Evolutionary Psychology: Innovative Research Strategies. Dordrecht: Springer
See also Shermer, Michael (2001). The Borderlands of Science: Where Sense Meets Nonsense. New York: Oxford University Press USA
- Harris, Judith R. (2006) No Two Alike: Human Nature And Human Individuality. New York: W.W. Norton