by Bryan Caplan
68 Less than a third of Americans believe that premarital sex is always wrong: General Social Survey 2008. Variable identifier PREMARS1.
68 if you were in the 80th percentile, you could expect your adopted sibling: J. Bailey et al., “Do Individual Differences in Sociosexuality Represent Genetic or Environmentally Contingent Strategies? Evidence from the Australian Twin Registry,” Journal of Personality and Social Psychology 78 (3) (March 2000), pp. 537–545.
69 A second study of 1,600 female twins in the United Kingdom: Lynn Cherkas et al., “Genetic Influences on Female Infidelity and Number of Sexual Partners in Humans: A Linkage and Association Study of the Role of the Vasopressin Receptor Gene (AVPR1A),” Twin Research 7 (6) (December 2004), pp. 649–658.
69 Psychologists used to label homosexuality: See Ronald Bayer, Homosexuality and American Psychiatry: The Politics of Diagnosis (New York: Basic Books, 1981).
69 identical twins are more alike in their sexual orientation than fraternal twins: Khytam Dawood et al., “Genetic and Environmental Influences on Sexual Orientation,” in Yong-Kyu Kim, ed., Handbook of Behavior Genetics (New York: Springer, 2009), p. 271. High-quality recent studies that confirm the standard findings include Katarina Alanko et al., “Common Genetic Effects of Gender Atypical Behavior in Childhood and Sexual Orientation in Adulthood: A Study of Finnish Twins,” Archives of Sexual Behavior 39 (1) (February 2010), pp. 81–92; and Niklas Långström et al., “Genetic and Environmental Effects on Same-Sex Sexual Behavior: A Population Study of Twins in Sweden,” Archives of Sexual Behavior 39 (1) (February 2010), pp. 75–80.
69 adopted brothers of gay men and adopted sisters of gay women: Dawood et al., “Genetic and Environmental Influences on Sexual Orientation,” p. 271.
69 A large Swedish twin study . . . confirms a small nurture effect: Långström et al., “Genetic and Environmental Effects on Same-Sex Sexual Behavior.”
69 “The princess realized that in the process of getting to know each other”: Leo Tolstoy, Anna Karenina (New York: Barnes and Noble, 1993), p. 42.
70 A study of over 4,000 Minnesota twins, most in their thirties and forties: Wendy Johnson et al., “Marriage and Personality: A Genetic Analysis,” Journal of Personality and Social Psychology 86 (2) (February 2004), pp. 285–294.
70 A long-running study of almost 6,000 men from the World War II Twin Registry: Susan Trumbetta et al., “Marriage and Genetic Variation Across the Lifespan: Not a Steady Relationship?” Behavior Genetics 37 (2) (March 2007), pp. 362–375. However, moderate family influence strangely reemerged when the twins were sixty and seventy.
70 A research team asked 1,000 female Swedish twins and their spouses: Eric Spotts et al., “Genetic and Environmental Influences on Marital Relationships,” Journal of Family Psychology 18 (1) (March 2004), pp. 107–119.
70 An early study using the Minnesota Twin Registry found large effects of genes: Matt McGue and David Lykken, “Genetic Influence on Risk of Divorce,” Psychological Science 3 (6) (November 1992), pp. 368–373.
70 Heredity matters a lot, upbringing doesn’t matter at all: Victor Jockin et al., “Personality and Divorce: A Genetic Analysis,” Journal of Personality and Social Psychology 71 (2) (August 1996), pp. 288–299.
70 A major study of Danish twins born 1870–1910: Hans-Peter Kohler et al., “Is Fertility Behavior in Our Genes? Findings from a Danish Study,” Population and Development Review 25 (2) (June 1999), pp. 253–288.
70 Upbringing had a tiny influence on when Danes tried to start a family: Joseph Rodgers et al., “Behavior Genetic Modeling of Human Fertility: Findings from a Contemporary Danish Twin Study,” Demography 38 (1) (February 2001), p. 36.
70 A different team of researchers looked at about 2,000 American twins: Joseph Rodgers and Debby Doughty, “Genetic and Environmental Influences on Fertility Expectations and Outcomes Using NLSY Kinship Data,” in Joseph Rodgers et al., Genetic Influences on Human Fertility and Sexuality (New York: Springer, 2000).
71 Siblings gave fairly similar answers whether or not they were biologically related: Kirby Deater-Deckard et al., “A Genetic Study of the Family Environment in the Transition to Early Adolescence,” Journal of Child Psychology and Psychiatry 40 (5) (July 1999), p. 772. The survey also found, however, that parents had little effect on how their children perceived their families’ achievement orientation.
71 The first questioned about 1,200 Minnesota twins: Matt McGue et al., “Perceptions of the Parent-Adolescent Relationship: A Longitudinal Investigation,” Developmental Psychology 41 (6) (November 2005), pp. 971–984.
71 They confirmed that parents continue to have moderate effects on how their young adult children: Susan South et al., “Adolescent Personality Moderates Genetic and Environmental Influences on Relationships with Parents,” Journal of Personality and Social Psychology 94 (5) (May 2008), pp. 899–912.
72 The Swedish study discovered moderate nurture effects for seven out of eight measures: Robert Plomin et al., “Genetic Influence on Childhood Family Environment Perceived Retrospectively from the Last Half of the Life Span,” Developmental Psychology 24 (5) (September 1988), p. 743.
72 Other studies of German, Canadian, and Swedish twins: Christian Kandler et al., “Genetic and Environmental Mediation Between Measures of Personality and Family Environment in Twins Reared Together,” Behavior Genetics 39 (1) (January 2009), pp. 24–35; Philip Vernon et al., “Environmental Predictors of Personality Differences: A Twin and Sibling Study,” Journal of Personality and Social Psychology 72 (1) (January 1997), pp. 177–183; Paul Lichtenstein et al., “Remembered Parental Bonding in Adult Twins: Genetic and Environmental Influences,” Behavior Genetics 33 (4) (July 2003), pp. 397–408.
72 “Extraordinary claims require extraordinary evidence”: Carl Sagan, Cosmos, Episode 12, “Encyclopedia Galactica.”
73 I take advantage of two mathematical facts implied by standard behavioral genetic models: See, for example, Robert Plomin et al., Behavioral Genetics (New York: Worth Publishers, 2008), pp. 374–402.
74 Suppose the trait in question is education: Baker et al., “Genetics of Educational Attainment in Australian Twins,” Behavior Genetics 26 (2) (March 1996), pp. 89–102.
74 If they report only correlations for identical versus fraternal twins: See, for example, Plomin et al., Behavioral Genetics, p. 382.
CHAPTER 3
77 prominent behavioral geneticists looked at major adoption studies of IQ: Matt McGue et al., “Behavioral Genetics of Cognitive Ability: A Life-Span Perspective,” in Robert Plomin and Gerald McClearn, eds., Nature, Nurture, and Psychology (Washington, DC: American Psychological Association, 1993), pp. 64, 67. For an unusually large twin study (11,000 subjects from four nations) of declining parental influence on IQ, see C. Haworth et al., “The Heritability of General Cognitive Ability Increases Linearly from Childhood to Young Adulthood,” Molecular Psychiatry (June 2009), available at www.nature.com/mp/journal/vaop/ncurrent/full/mp200955a.html.
78 The Colorado Adoption Project provides an especially vivid illustration: Plomin et al., “Nature, Nurture, and Cognitive Development.”
78 “Adopted children resemble their adoptive parents slightly in early childhood”: Ibid., p. 442.
78 One study of over 5,000 Swedish twins: David Cesarini, “The Effect of Family Environment on Productive Skills, Human Capital, and Lifetime Income,” MIT Working Paper, 2010.
78 there are noticeable nurture effects for children younger than fifteen: Jacobson et al., “Sex Differences in the Genetic and Environmental Influences on the Development of Antisocial Behavior,” p. 409.
78 A major review of twin and adoption studies of antisocial and criminal behavior: Rhee and Waldman, “Genetic and Environmental Influences on Antisocial Behavior: A Meta-Analysis of Twin and Adoption Studies,” p. 514.
79 Parents influence when their daughters start having sex: J. Bailey et al., “Do Individual Differences in Sociosexuality Represent Genetic or Environmentally Contingent Strategies?” S
ee also Cherkas et al., “Genetic Influences on Female Infidelity and Number of Sexual Partners in Humans.”
79 The cleanest study asked over 500 Minnesota twins questions: Koenig et al., “Genetic and Environmental Influences on Religiousness.”
82 Because there’s a lot more to “the environment” than the family: For further discussion, see Harris, No Two Alike.
83 If this assumption is wrong, standard models oversell the effect of nature: See D. A. Grayson, “Twins Reared Together: Minimizing Shared Environmental Effects,” Behavior Genetics 19 (4) (December 1989), pp. 593–604.
83 Cutting-edge studies of the children of twins find bigger effects: See, for example, Brian D’Onofrio et al., “Intergenerational Transmission of Childhood Conduct Problems: A Children of Twins Study,” Archives of General Psychiatry 64 (7) (July 2007), pp. 820–829. ; Brian D’Onofrio et al., “A Children of Twins Study of Parental Divorce and Offspring Psychopathology,” Journal of Child Psychology and Psychiatry 48 (7) (July 2007), pp. 667–675; Brian D’Onofrio et al., “A Genetically Informed Study of the Intergenerational Transmission of Marital Instability,” Journal of Marriage and Family 69 (3) (August 2007), pp. 793–809 ; and Stacy Lynch et al., “A Genetically Informed Study of the Association Between Harsh Punishment and Offspring Behavioral Problems,” Journal of Family Psychology 20 (2) (June 2006), pp. 190–198. The interpretation of children of twins studies remains problematic. As one prominent paper admits, “the Children of Twins Design does not control for genetic risk from the spouse of twins” (D’Onofrio et al., “A Genetically Informed Study of the Intergenerational Transmission of Marital Instability,” p. 806). For further discussion, see Lindon Eaves et al., “Revisiting the Children of Twins: Can They Be Used to Resolve the Environmental Effects of Dyadic Parental Treatment on Child Behavior?” Twin Research and Human Genetics 8 (4) (August 2005), pp. 283–290.
83 Twins come from all walks of life: See, for example, Eric Turkheimer et al., “Socioeconomic Status Modifies Heritability of IQ in Young Children,” Psychological Science 14 (6) (November 2003), pp. 623–628. Twin studies based on national registries don’t have this problem, but countries with national registries tend to have less inequality than the United States.
85 the median American went to church “several times a year”: General Social Survey, 2008. Variable identifier ATTEND.
85 parents suffer from what psychologists call the illusion of control: See, for example, Nathanael Fast et al., “Illusory Control: A Generative Force Behind Power’s Far-Reaching Effects,” Psychological Science 20 (4) (April 2009), pp. 502–508.
87 For personality traits, however, spouses are barely alike: See David Lykken and Auke Tellegen, “Is Human Mating Adventitious or the Result of Lawful Choice? A Twin Study of Mate Selection,” Journal of Personality and Social Psychology 65 (1) (July 1993), pp. 56–68; Robert Krueger et al., “Assortative Mating for Antisocial Behavior: Developmental and Methodological Implications,” Behavior Genetics 28 (3) (March 1998), pp. 173–186; J. Rushton, “Genetic Similarity, Human Altruism, and Group Selection,” Behavioral and Brain Sciences 12 (3) (1989), pp. 503–559; Sven Wilson, “The Health Capital of Families: An Investigation of the Inter-Spousal Correlation in Health Status,” Social Science and Medicine 55 (7) (October 2002), pp. 1157–1172; and Ulrich Schimmack and Richard Lucas, “Marriage Matters: Spousal Similarity in Life Satisfaction,” Schmollers Jahrbuch 127 (1) (January 2007), pp. 105–111. Admittedly, low personality correlations could instead show that many seek mates who differ from them. But when some traits are widely seen as better than others, why would opposites attract?
87 Over 13 percent of the children in Malawi: For child mortality in Malawi, see World Population Prospects: The 2006 Revision (New York: United Nations, 2007), Table A.19. Madonna successfully appealed the initial rejection of her adoption petition. See Larry McShane, “Madonna Malawi Adoption Request Rejected By Court,” New York Daily News, April 3, 2009; and Raphael Tenthani, “Malawi Welcomes Madonna Adoption,” BBC News, June 13, 2009.
87 Merely moving an adult Nigerian to the United States: Michael Clemens et al., “The Place Premium: Wage Differences for Identical Workers Across the U.S. Border,” Washington, DC: Center for Global Development Working Paper, 2008.
87 There are even waiting lists for domestic adoption of special needs children: Heidi Lindh, “Characteristics and Perspectives of Families Waiting to Adopt a Child with Down Syndrome,” Genetics in Medicine 9 (4) (April 2007), pp. 235–240.
88 “So you mean it doesn’t matter how I treat my child?”: Harris, The Nurture Assumption, p. 341.
CHAPTER 4
94 They’re straight out of the original document: If you’re curious about the details, see the Appendix to Chap. 4: Where the Mortality Tables Come From.
100 Whenever Gallup asks, “Is there more crime in the U.S.”: Lydia Saad, “Perceptions of Crime Remain Curiously Negative,” available at www.gallup.com/poll/102262/perceptions-crime-problem-remain-curiously-negative.aspx, October 22, 2007.
100 In 2008, it estimated a violent crime rate of 19.3 per 1,000: Michael Rand, “Criminal Victimization, 2008,” Bureau of Justice Statistics Bulletin (September 2009), Table 3, available at bjs.ojp.usdoj.gov/content/pub/pdf/cv08.pdf. The NCVS does not measure murder because (a) it is based on interviews with victims, and (b) murder is almost always reported anyway.
101 But according to police reports, kids under twelve are much safer: David Finkelhor and Richard Ormrod, “Characteristics of Crimes Against Juveniles,” Office of Juvenile Justice and Delinquency Prevention (2000), available at www.ncjrs.gov/pdffiles1/ojjdp/179034.pdf.
101 The best source on child abduction: Andrea Sedlak et al., “National Estimates of Missing Children: An Overview,” Office of Juvenile Justice and Delinquency Prevention (2002), available at www.ncjrs.gov/pdffiles/ojjdp/196465.pdf.
103 no child has ever been killed or seriously injured by Halloween treats: See Joel Best, Threatened Children: Rhetoric and Concern About Child-Victims (Chicago: University of Chicago Press, 1993), and his more recent update, “Halloween Sadism: The Evidence,” University of Delaware Library Institutional Repository (2008), available at dspace.udel.edu:8080/dspace/handle/19716/726.
104 “Once you can picture an eight-year-old”: Skenazy, Free-Range Kids, p. 43.
104 One of psychologists’ most effective treatments for anxiety: See David Richard and Dean Lauterbach, eds., Handbook of Exposure Therapies (Burlington, MA: Academic Press, 2006).
106 During 1950, 14,650 soldiers died in the Korean War: For the Korean War, age breakdowns are only available for the army. I assumed that the age breakdown for the other armed services was equal to the army’s.
106 To estimate war deaths for 2005, I assumed that the age breakdown: Military fatality statistics for Operation Enduring Freedom and Operation Iraqi Freedom break down deaths into five age categories: under 22, 22–24, 25–30, 31–35, and over 35. My estimates in Table 4.1 assume that 20 percent of all fatalities for those 31–35 are 35-year-olds, and 100 percent of 36+ fatalities fall in the 35–44 bracket.
CHAPTER 5
109 Becker saw economics in the family: See Gary Becker, A Treatise on the Family (Cambridge, MA: Harvard University Press 1993).
110 The greatest perceived triumph of the economics of the family: All total fertility rates are from “Total Fertility Rate (children per woman),” available at www.UNdata.org.
111 A variant on the rising-women’s-wages story claims: See, for example, James Feyrer et al., “Will the Stork Return to Europe and Japan? Understanding Fertility Within Developed Nations,” Journal of Economic Perspectives 22 (3) (June 2008), pp. 3–22.
111 When the World Values Survey queried men and women around the world: World Values Survey, Variable identifier D017, available at www.worldvaluessurvey.org.
111 Condoms have been widely available since World War II: Aine Collier, The Humble Little Condom (New York: Prometheus Books, 2007), pp. 236–238.
112 “Children consumed more food than they caught”: Theodore Bergstrom, “Economics in a Family Way,” Journal of Economic Literature 34 (4) (Spring 1996), p. 1913.
112 Investing in your children is less lucrative than: Ibid., p. 1914.
112 One research team using the Survey of Consumer Finances: William Gale and John Scholz, “Intergenerational Transfers and Accumulation of Wealth,” Journal of Economic Perspectives 8 (4) (Fall 1994), p. 149.
112 parents on average voluntarily gave a total of $25,000 more: Ronald Lee, “Population Age Structure, Intergenerational Transfer, and Wealth: A New Approach, with Applications to the United States,” Journal of Human Resources 29 (4) (1994), pp. 1051–1052.
112 At the time, women earned 60 percent as much as men: “Table 2. Employment Status of the Civilian Noninstitutional Population 16 Years and Over by Sex, 1970–2004 Annual Averages,” Bureau of Labor Statistics, available at www.bls.gov/cps/wlf-table2–2006.pdf; Francine Blau and Lawrence Kahn, “The U.S. Gender Pay Gap in the 1990s: Slowing Convergence,” Industrial and Labor Relations Review 60 (1) (June 2006), pp. 45–66.