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Offspring: Human Fertility Behavior in Biodemographic Perspective
sions and processes points to a broad framework of genetically mediated influences. In particular, differential genetic dispositions can exert influences on fertility and related behaviors through at least three distinct pathways. On the one hand, biological dispositions affect fertility relatively directly through genetically mediated variations in physiological characteristics affecting fertility outcomes. Genetic influences on fecundity are an obvious example (e.g., see Christensen et al., 2003), but there are also other possibilities, including, for instance, the fact that physical characteristics might render a person especially attractive in the marriage market, which increases the probability of an early marriage because of an unexpected high frequency of attractive marriage offers in early adulthood.
On the other hand, and potentially more interesting in the context of this paper, biological dispositions affect fertility through deliberate fertility decisions and a broad range of fertility-related behaviors that are subject to substantial volitional control. Within this category of influences, we can further distinguish between two different pathways. First, some biological dispositions exert their effect on behavior through conscious decision making and life course planning. Second, biological dispositions may also operate subconsciously on decision processes if individuals are not aware of their background influences on aspects such as emotions, preferences, or cognitive abilities. Examples for the former are individuals’ knowledge about their fecundity (e.g., see Rosenzweig and Schultz, 1985) or knowledge about their returns to schooling and delaying fertility (see, for instance, Behrman et al., 1994, 1996). In addition, Halpern et al. (2000) found that “smart teens don’t have sex or kiss much either,” which is consistent with higher cognitive abilities and an awareness about the high costs of early pregnancies due to foregone opportunities. Further examples of subconscious influences are variations in evolved preferences for nurturing (Foster, 2000; Miller and Rodgers, 2001). Moreover, early sexual activity, which is a predictor of early fertility, has also been related to nonvolitional factors such as hormone levels and body fat (e.g., Halpern et al., 1997, 1999), both of which are subject to strong genetic variations.
In summary, the above interpretations view genetic dispositions as part of individuals’ endowments that affect their life course patterns, including those pertaining to fertility and related behaviors, through their effect on (1) conscious decision making and deliberate life cycle planning, (2) nonvolitional processes affecting life course outcomes, and (3) physical characteristics or cognitive abilities that partially determine opportunities in the labor market and marriage market. Instead of focusing on these specific pathways of how genetic dispositions affect fertility outcomes, most current behavioral genetics designs try to identify and estimate the net contribution of a broad range of genetically mediated biological factors on the variations in fertility behavior within a population or within cohorts.