Multivariate Cholesky Models of Human Female Fertility Patterns in the NLSY

Abstract

Substantial evidence now exists that variables measuring or correlated with human fertility outcomes have a heritable component. In this study, we define a series of age-sequenced fertility variables, and fit multivariate models to account for underlying shared genetic and environmental sources of variance. We make predictions based on a theory developed by Udry [(1996) Biosocial models of low-fertility societies. In: Casterline, JB, Lee RD, Foote KA (eds) Fertility in the United States: new patterns, new theories. The Population Council, New York] suggesting that biological/genetic motivations can be more easily realized and measured in settings in which fertility choices are available. Udry's theory, along with principles from molecular genetics and certain tenets of life history theory, allow us to make specific predictions about biometrical patterns across age. Consistent with predictions, our results suggest that there are different sources of genetic influence on fertility variance at early compared to later ages, but that there is only one source of shared environmental influence that occurs at early ages. These patterns are suggestive of the types of gene-gene and geneenvironment interactions for which we must account to better understand individual differences in fertility outcomes. KeywordsFertility; Fisher's theorem; FTNS; Heritability; Shared Environment; Multivariate models; Phenotypic plasticity; Hox genes; Life history theory Fisher's (1930) Fundamental Theorem of Natural Selection (the FTNS) has been misinterpreted by many researchers for many years. The traditional interpretation is that fitness traits and behaviors strongly affected by natural selection will "lose" their genetic variance in the long run. The implication is that fertility and fertility precursors should have little or no genetic variance, and thus zero heritability. This inference is correct if natural selection is the only process at work. But it is not. The FTNS has been interpreted by many to mean that, by definition, for fertility (and other traits related to fitness), h 2 = 0. But it does not. Hughes and Burleson (2000) suggested a number of different processes that re-introduce genetic variance into fitness traits, even while natural selection is washing it out. These include NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript mutation (the most important), frequency-dependent selection, heterozygote advantage (overdominance), sexual antagonism, and environmental perturbations. Fisher (1930) clearly did not intend or participate in this misinterpretation of the FTNS. In the same source in which he presented the FTNS, he also discussed the role of contraception (an environmental perturbation) in re-introducing genetic variance into human fertility (see also Houle 1992, for additional and more modern discussion of "perturbing forces" from the environment). Further, Fisher presented empirical data suggesting significant heritability in family size among the British aristocracy, h 2 = 0.40. Eve...

Yearning, Learning and Conceding: (Some of) the Reasons People Change Their Childbearing Intentions

“…We experimented with cut points for this variable ranging from age 29 to age 44; for men, this variable was never significant, whereas for women, the only formulation which provided a significant coefficient was a variable indicating if the individual was aged 30 or over -this is in line with the results of (Rodgers et al 2007) which suggest that until the age of 30 women do not have yet concerns over their upper biological limits ; even in this case, the coefficient was relatively small compared to the size of the coefficient on age. These estimates are presented in Column 1 as Specification V; they provide some evidence, although fairly weak, in favour of Hypotheses 5 and 5b, namely that the probability of downward revisions does increase as women approach the end of their fertile years.…”

Genetic influence on age at first birth of female twins born in the UK, 1919–68

“…The state of the social environment also appears to affect the extent to which biological predispositions affect the processes of decision-making (Udry 1996;Rodgers et al 2007). Thus, research that has revealed large changes in the extent of genetic influences on fertility suggests that the main reason for the changes has been variation in the extent to which people have felt free to choose whether and when to become a parent and in what familial circumstances.…”

Education and Cognitive Ability as Direct, Mediating, or Spurious Influences on Female Age at First Birth: Behavior Genetic Models Fit to Danish Twin Data

“…The simple fact that we (as well as Neiss et al [2002]) found relatively small heritability in AFB is one example of this. Recent studies have documented important and systematic genetic variance underlying many other fertility variables, including age at first attempt to get pregnant (Rodgers, Kohler, et al 2001), ever having a child (Kohler et al 1999), fertility motivation (Rodgers and Doughty 2000), age at first intercourse (Miller et al 1999;Rodgers, Rowe, and Buster 1999), and marriage (Trumbetta and Gottesman 2000), and there have been a number of studies of direct fertility variables such as completed family size (Kohler et al 1999;Rodgers, Kohler, et al 2001;Christensen et al 2003;Rodgers, Bard, and Miller 2007;see Rodgers, Kohler, et al 2001 andKohler et al 2006 for summaries). One partial explanation for this lower heritability is that AFB may have substantially more random variation contained in its measure than other fertility variables.…”

“…First, the timing of the second Danish demographic transition coincided with the timing of childbearing during this cohort, and fertility choice theory (Udry 1996;Rodgers et al 2007) links declining fertility with the release of latent genetic variance. Second, a number of significant univariate heritabilities have been identified in relation to other fertility variables in the Danish twins and more generally (Kohler et al 1999(Kohler et al , 2002Rodgers, Hughes, et al 2001).…”

“…Second, a number of significant univariate heritabilities have been identified in relation to other fertility variables in the Danish twins and more generally (Kohler et al 1999(Kohler et al , 2002Rodgers, Hughes, et al 2001). Third, multivariate relationships have been identified among fertility variables from this period and slightly later (Rodgers, Kohler, et al 2001;Rodgers et al 2007). But there was no genetic variance identified, even in a setting in which theory and past empirical results suggested there would likely be such variance.…”