Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think

by Bryan Caplan

  48 A major study of adult Swedish twins looked at gingivitis: L. A. Mucci et al., “Environmental and Heritable Factors in the Etiology of Oral Diseases: A Population-Based Study of Swedish Twins,” Journal of Dental Research 84 (9) (September 2005), pp. 800–805.

  49 A study of over 3,000 male twins from the Vietnam Era Twin Registry: William True et al., “Common Genetic Vulnerability for Nicotine and Alcohol Dependence in Men,” Archives of General Psychiatry 56 (7) (July 1999), pp. 655–661.

  49 another study of the same group of twins found that nurture mattered: Ming Tsuang et al., “Genetic and Environmental Influences on Transitions in Drug Use,” Behavior Genetics 29 (6) (December 1999), pp. 473–479.

  49 A study of Australian twins found: A. C. Heath et al., “Genetic and Environmental Contributions to Alcohol Dependence Risk in a National Twin Sample: Consistency in Findings of Women and Men,” Psychological Medicine 27 (6) (November 1997), pp. 1381–1396.

  49 Researchers using the Virginia 30,000 sample: Hermine Maes et al., “Genetic and Cultural Transmission of Smoking Initiation: An Extended Twin Kinship Model,” Behavior Genetics 36 (6) (2006), pp. 795–808.

  49 One team studied the tobacco, alcohol, and drug use: Cong Han et al., “Lifetime Tobacco, Alcohol and Other Substance Use in Adolescent Minnesota Twins: Univariate and Multivariate Behavioral Genetic Analyses,” Addiction 94 (7) (July 1999), pp. 981–993.

  49 other studies conclude that parents don’t affect drinking: Matt McGue, “The Behavioral Genetics of Alcoholism,” Current Directions in Psychological Science 8 (4) (August 1999), p. 113.

  49 adoptees drank like their adopted siblings: Matt McGue et al., “Parent and Sibling Influences on Adolescent Alcohol Use and Misuse: Evidence from a U.S. Adoption Cohort,” Journal of Studies on Alcohol 57 (1) (January 1996), pp. 8–18.

  49 study of over 1,000 Koreans adopted by American families: Bruce Sacerdote, “How Large Are the Effects from Changes in Family Environment? A Study of Korean American Adoptees,” Quarterly Journal of Economics 122 (1) (January 2007), p. 143.

  50 One misreported study about “the Mozart effect”: Marina Krakowsky, “Discredited ‘Mozart Effect’ Remains Music to American Ears,” Stanford Graduate School of Business News (February 2005).

  50 The Minnesota Study of Twins Reared Apart reunited almost 100: Thomas Bouchard et al., “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart,” Science 250 (4978) (October 1990), p. 223. The first test was the Wechsler Adult Intelligence Scale. The second test was a composite of the Raven Progressive Matrices and the Mill-Hill Vocabulary Test.

  50 If you did better than 80 percent of the population on both tests: Ibid., p. 226.

  50 “Growing up in the same family does not contribute to similarity”: Robert Plomin et al., “Variability and Stability in Cognitive Abilities Are Largely Genetic Later in Life,” Behavior Genetics 24 (3) (1994), p. 214.

  50 tested about 1,600 reared-together adult twins from the Dutch Twin Registry: Daniëlle Postuma et al., “Genetic Contributions to Anatomical, Behavioral, and Neurophysiological Indices of Cognition,” in Robert Plomin et al., eds., Behavioral Genetics in the Postgenomic Era (Washington, DC: American Psychological Association, 2002), p. 143.

  51 In 1975, the Colorado Adoption Project began studying 245 adopted babies: Robert Plomin et al., “Nature, Nurture, and Cognitive Development from 1 to 16 Years: A Parent-Offspring Adoption Study,” Psychological Science 8 (6) (November 1997), pp. 442–447.

  51 The Texas Adoption Project . . . also found no effect of upbringing on the IQs: John Loehlin et al., “Heredity, Environment, and IQ in the Texas Adoption Project,” in Robert Sternberg and Elena Grigorenko, eds., Intelligence, Heredity, and Environment (Cambridge: Cambridge University Press, 1997), p. 123.

  51 If you’re happier than 80 percent of people, your fraternal twin: David Lykken, Happiness: The Nature and Nurture of Joy and Contentment (New York: St. Martin’s Press, 2000), p. 57.

  52 “Nearly 100 percent of the variation in the happiness set point”: Ibid., p. 58.

  52 Twins raised apart were more alike in happiness: Ibid., p. 56.

  52 The single most impressive study interviewed almost 8,000 twins: K. S. Kendler et al., “A Population-Based Twin Study of Self-Esteem and Gender,” Psychological Medicine 28 (6) (June 1998), p. 1406.

  52 further research using the Minnesota Twin Registry concludes: Deborah Finkel and Matt McGue, “Sex Differences and Nonadditivity in Heritability of the Multidimensional Personality Questionnaire Scales,” Journal of Personality and Social Psychology 72 (4) (April 1997), pp. 929–938.

  54 In 2004–2005, economist Bruce Sacerdote tracked down: Sacerdote, “How Large Are the Effects from Changes in Family Environment?”

  54 Another major study of over 2,000 Swedish adoptees: Anders Björklund et al., “The Origins of Intergenerational Associations: Lessons from Swedish Adoption Data,” Quarterly Journal of Economics 129 (3) (August 2006), p. 1013.

  54 The most remarkable examined about 2,000 pairs of American twins: Jere Behrman and Paul Taubman, “Is Schooling Mostly ‘In the Genes’? Nature-Nurture Decomposition Using Data on Relatives,” Journal of Political Economy 97 (6), pp. 1425–1446.

  55 Researchers who looked at about 2,500 Australian twins: Paul Miller et al., “Genetic and Environmental Contributions to Educational Attainment in Australia,” Economics of Education Review 20 (3) (June 2001), pp. 211–224; see also Paul Miller et al., “What Do Twin Studies Reveal About the Economic Returns to Education? A Comparison of Australian and U.S. Findings,” American Economic Review 85 (3) (June 1995), pp. 586–599.

  55 An early study of Norwegian twins found strong family effects: A. C. Heath et al., “Education Policy and the Heritability of Educational Attainment,” Nature 314 (6013) (April–May 1985), pp. 734–736.

  55 Researchers using the Minnesota Twin Family Registry and the Finnish Twin Cohort Study: Karri Silventoinen et al., “Heritability of Body Height and Educational Attainment in an International Context: Comparison of Adult Twins in Minnesota and Finland,” American Journal of Human Biology 16 (5) (September–October 2004), pp. 544–555.

  55 One team combined thousands of observations from earlier studies: George Vogler and David Fulker, “Familial Resemblance for Educational Attainment,” Behavior Genetics 13 (4) (July 1983), pp. 341–354.

  56 A more recent study of Australian twins reports moderate nurture effects: Laura Baker et al., “Genetics of Educational Attainment in Australian Twins,” Behavior Genetics 26 (2) (March 1996), pp. 89–102.

  56 An early study of about 500 Australian twins reported little or no effect of upbringing: C. E. Gill et al., “Further Evidence for Genetic Influences on Educational Achievement,” British Journal of Educational Psychology 55 (3) (November 1985), pp. 240–250. This study did, however, note that only the top 34 percent of students took the exam and argued that nurture would matter more for the bottom two-thirds of the ability distribution.

  56 A research team investigating the attitudes of almost 700 Canadian twins: James Olson et al., “The Heritability of Attitudes: A Study of Twins,” Journal of Personality and Social Psychology 80 (6) (June 2001), p. 850.

  56 The most impressive evidence . . . comes from the U.S.-based National Longitudinal Study of Adolescent Health: François Nielsen, “Achievement and Ascription in Educational Attainment: Genetic and Environmental Influences on Adolescent Schooling,” Social Forces 85 (1) (September 2006), pp. 193–216.

  57 In Sacerdote’s Korean adoption study, biological children from richer families: Sacerdote, “How Large Are the Effects from Changes in Family Environment?” p. 142.

  57 The Swedish adoption study mentioned earlier finds small effects: Björklund et al., “The Origins of Intergenerational Associations,” p. 1013.

  57 Identical twins turn out to be almost exactly twice as similar in labor incomes: David Cesarini, “Decomposing the Genetic Variance in Income: The Role of Cognitive and Nonco
gnitive Skill,” MIT Working Paper, 2010.

  57 A study of over 2,000 Australian twins finds the same thing: Miller et al., “What Do Twins Studies Reveal About the Economic Returns to Education?” p. 590.

  58 In the U.S. Twinsburg Study: Samuel Bowles and Herbert Gintis, “The Inheritance of Inequality,” Journal of Economic Perspectives 16 (3) (Summer 2002), p. 16.

  58 A study of American full and half siblings using the National Longitudinal Survey of Youth: David Rowe et al., “Herrnstein’s Syllogism: Genetic and Shared Environmental Influences on IQ, Education, and Income,” Intelligence 26 (4) (November 1998), p. 419.

  59 If you make a list of the traits that almost all parents want to instill: See, for example, Ralph Piedmont, The Revised NEO Personality Inventory: Clinical and Research Applications (New York: Plenum Press, 1998); and Robert Hogan et al., eds., Handbook of Personality Psychology (New York: Academic Press, 1997). The agreeableness umbrella also covers some seemingly undesirable cognitive traits such as “illogical” and “emotional.”

  60 One of the earliest studies of nature, nurture, and character: C. S. Bergeman et al., “Genetic and Environmental Effects on Openness to Experience, Agreeableness, and Conscientiousness: An Adoption/Twin Study,” Journal of Personality 61 (2) (June 1993), pp. 159–179.

  60 A survey article published in Science a year later: Thomas Bouchard, “Genes, Environment, and Personality,” Science 264 (5166) (June 1994), pp. 1700–1701.

  60 One team looked at almost 2,000 German twins: Rainer Riemann et al., “Genetic and Environmental Influences on Personality: A Study of Twins Reared Together Using the Self- and Peer Report NEO-FFI Scales,” Journal of Personality 65 (3) (September 1997), pp. 449–475.

  60 Studies of 1,600 American twins and 600 Canadian twins: John Loehlin et al., “Heritabilities of Common and Measure-Specific Components of the Big Five Personality Factors,” Journal of Research in Personality 32 (4) (December 1998), pp. 431–453; Kerry Jang et al., “Heritability of the Big Five Personality Dimensions and Their Facets: A Twin Study,” Journal of Personality 64 (3) (September 1996), pp. 577–591.

  60 you might prefer the Swedish approach: Cesarini, “Decomposing the Genetic Variance in Income.”

  61 If your parents were higher in conscientiousness and agreeableness than 80 percent of the population: John Loehlin, “Resemblance in Personality and Attitudes Between Parents and Their Children,” in Samuel Bowles et al., eds., Unequal Chances: Family Background and Economic Success (Princeton: Princeton University Press, 2005), p. 198.

  61 In 1984, Science published a study of almost 15,000 Danish adoptees: Sarnoff Mednick et al., “Genetic Influences in Criminal Convictions: Evidence from an Adoption Cohort,” Science 224 (4651) (May 1984), pp. 891–894.

  62 In 2002, a study of antisocial behavior in almost 7,000 Virginian twins: Kristen Jacobson et al., “Sex Differences in the Genetic and Environmental Influences on the Development of Antisocial Behavior,” Development and Psychopathology 14 (2) (Spring 2002), p. 412.

  62 a major review of fifty-one twin and adoption studies reported: Soo Rhee and Irwin Waldman, “Genetic and Environmental Influences on Antisocial Behavior: A Meta-Analysis of Twin and Adoption Studies,” Psychological Bulletin 128 (3) (May 2002), pp. 512–513.

  63 A major study of over 7,000 adult Australian twins: Brian D’Onofrio et al., “Understanding Biological and Social Influences on Religious Affiliation, Attitudes, and Behaviors: A Behavior Genetic Perspective,” Journal of Personality 67 (6) (December 1999), pp. 953–984.

  64 Another study of almost 2,000 women from the Virginia Twin Registry: Kenneth Kendler et al., “Religion, Psychopathology, and Substance Use and Abuse: A Multimeasure, Genetic-Epidemiologic Study,” American Journal of Psychiatry 154 (3) (March 1997), p. 325.

  64 One early study of almost 2,000 adult Minnesota twins reared together and apart: Niels Waller et al., “Genetic and Environmental Influences on Religious Interests, Attitudes, and Values: A Study of Twins Reared Apart and Together,” Psychological Science 1 (2) (March 1990), pp. 138–142.

  64 A recent follow-up found similar results: Thomas Bouchard et al., “Genetic Influence on Social Attitudes: Another Challenge to Psychology from Behavior Genetics,” in Lisabeth DiLalla, ed., Behavior Genetics Principles: Perspectives in Development, Personality, and Psychopathology (Washington, DC: American Psychological Association, 2004), pp. 92–95.

  64 Parents have almost no effect on adult church attendance: D’Onofrio et al., “Understanding Biological and Social Influences on Religious Affiliation, Attitudes, and Behaviors.” They did, however, find that twins had a noticeable effect on each other’s church attendance.

  64 Another major study of American and Australian church attendance: K. M. Kirk et al., “Frequency of Church Attendance in Australia and the United States: Models of Family Resemblance,” Twin Research 2 (2) (June 1999), pp. 99–107.

  65 If you’re more religious than 80 percent of people: Laura Koenig et al., “Genetic and Environmental Influences on Religiousness: Findings for Retrospective and Current Religiousness Ratings,” Journal of Personality 73 (2) (April 2005), pp. 471–488.

  65 When about 600 female Minnesota twins took the same test: Laura Koenig et al., “Stability and Change in Religiousness During Emerging Adulthood,” Developmental Psychology 44 (2) (March 2008), pp. 532–543.

  65 In the Virginia 30,000 study, both identical and fraternal twins: Peter Hatemi et al., “Is There a ‘Party’ in Your Genes?” Political Research Quarterly 62 (3) (September 2009), Tables 3, 5.

  65 Party identification works the same way in Australia: Peter Hatemi et al., “The Genetics of Voting: An Australian Twin Study,” Behavior Genetics 37 (3) (May 2007), pp. 435–448.

  65 The Virginia 30,000 study found that parents have little effect on the strength: Hatemi et al., “Is There a ‘Party’ in Your Genes?” Table 5.

  66 parents have little influence over whether people bother to vote: James Fowler et al., “Genetic Variation in Political Participation,” American Political Science Review 102 (2) (May 2008), pp. 233–248.

  66 Parents may slightly affect how conservative you are: For details, see Thomas Bouchard and Laura Koenig, “Genetic and Environmental Influences on the Traditional Moral Values Triad—Authoritarianism, Conservatism, and Religiousness—as Assessed by Quantitative Behavior Genetic Methods,” in Patrick McNamara, ed., Where God and Science Meet, vol. 1 (Westport, CT: Praeger Publishers, 2006); Thomas Bouchard and Matt McGue, “Genetic and Environmental Influences on Human Psychological Differences,” Journal of Neurobiology 52 (1) (January 2003), pp. 27–29; Lindon Eaves et al., “Comparing the Biological and Cultural Inheritance of Personality and Social Attitudes in the Virginia 30,000 Study of Twins and Their Relatives,” Twin Research 2 (2) (June 1999), pp. 62–80; Hatemi, “The Genetics of Voting”; John Alford et al., “Are Political Orientations Genetically Transmitted?” American Political Science Review 99 (2) (June 2005), pp. 153–167; Olson et al., “Heritability of Attitudes”; N. G. Martin et al., “Transmission of Social Attitudes,” Proceedings of the National Academy of Sciences 83 (12) (June 1986), pp. 4364–4368.

  67 including studies of over 1,000 Swedes raised apart and together: Bergman et al., “Genetic and Environmental Effects on Openness to Experience, Agreeableness, and Conscientiousness”; Riemann et al., “Genetic and Environmental Influences on Personality”; Loehlin et al., “Heritabilities of Common and Measure-Specific Components of the Big Five Personality Factors”; Jang et al., “Heritability of the Big Five Personality Dimensions and Their Facets.”

  67 The average adoption study finds a small but reliable effect of parenting: Loehlin, “Resemblance in Personality and Attitudes Between Parents and Their Children,” p. 198.

  67 The first included over 3,000 women: M. P. Dunne et al., “Genetic and Environmental Contributions to Variance in Age at First Sexual Intercourse,” Psychological Science 8 (3) (May 1997), pp. 211–216.

  67 A follow-up roughly doubl
ed the sample size: Mary Waldron et al., “Age at First Sexual Intercourse and Teenage Pregnancy in Australian Female Twins,” Twin Research and Human Genetics 10 (3) (June 2007), pp. 440–449.

  68 Another research team used the National Longitudinal Survey of Youth: Joseph Rodgers et al., “Nature, Nurture and First Sexual Intercourse in the USA: Fitting Behavioural Genetic Models to NLSY Kinship Data,” Journal of Biosocial Science 31 (1) (January 1999), pp. 29–41.

  68 Girls’ parents are more likely to take extreme measures: If fear of teen pregnancy merely increased the average effort of parents of girls, there would be no reason to expect differences in nurture to matter more. But as long as fear of teen pregnancy also increases the variation in parents’ effort, we should expect a larger nurture effect.

  68 While the Australian study of almost 7,000 female twins found: Waldron et al., “Age at First Sexual Intercourse and Teenage Pregnancy in Australian Female Twins.”

  68 A study of about 2,000 female Swedish twins born in the Fifties: Petra Olausson et al., “Aetiology of Teenage Childbearing: Reasons for Familial Effects,” Twin Research 3 (1) (March 2000), pp. 23–27.