It has long been recognized that females undergo a variety of hormonal changes during pregnancy and childbirth that may facilitate maternal behavior. Rosenblatt (1969, 1995, in Vol. 2 of this Handbook), using the rat as an experimental model, showed that hormonal changes elicited maternal behavior, while other studies showed similar effects for human mothers (Fleming, Ruble, Krieger, and Wong, 1997). It was long assumed that hormones play an unimportant role in paternal behavior because a father's exposure to rat pups increases paternal activity without any changes in hormone levels (Fleming and Corter, 1995). More recent evidence has challenged the assumption that hormonal levels are unimportant determinants of paternal behavior when this issue was examined in species other than the rat, which is not a natural paternal species. In naturally paternal species, such as canid species who constitute less than 10% of mammalian species (Storey, Walsh, Quinton, and Wynne-Edwards, 2000), researchers have found that in a variety of animal species males experience hormonal changes, including increases in prolactin and decreases in testosterone before the onset of parental behavior and during infant contact (Fleming and Corter, 1995; Rosenblatt, 1995). Human fathers, too, undergo hormonal changes during their mates' pregnancy and childbirth. Storey et al. (2000) found that men experienced significant prenatal, perinatal, and postnatal changes in each of these hormones—prolactin, cortisol, and testosterone—a pattern of results which was similar to that of the women in their study. Specifically, prolactin levels were higher for both men and women in the late prenatal period than in the early prenatal period, and cortisol levels increased just before birth and decreased in the postnatal period for both men and women. Testosterone levels were lower in the early postnatal period, which corresponds to the first opportunity for interaction with their infants. Hormonal levels and changes were linked with a variety of social stimuli as well. Men with lower testosterone levels held test baby dolls longer and were more responsive to infant cues (crying) than were men with higher testosterone levels. Men who reported a greater drop in testosterone levels also reported more pregnancy or couvade symptoms. Together these findings suggest that lower testosterone levels in the postnatal period may increase paternal responsiveness, in part by reducing competitive nonnurturing behavior (Storey et al., 2000). Similarly prolactin levels were higher in men who showed greater responsiveness to infant cries and in men who reported more couvade symptoms during their mates' pregnancies. Finally, Storey et al. (2000, p. 91) argued that the "cortisol increases in late pregnancy and during labor may help new fathers focus on and become attached to their newborns." Men's changes in hormonal levels are linked not only with baby cries and the time in the pregnancy birth cycle but also to the hormonal levels of their partners. Women's hormonal levels were closely linked with the time remaining before delivery, and men's levels were linked with their partner's hormone levels, not with time to birth. This suggests that contact with the pregnant partner may play a role in paternal responsiveness, just as the quality of the marital relationship is linked with paternal involvement in later infancy. This suggests that social variables need to be considered in understanding the operation of biological effects. Perhaps intimate ties between partners during pregnancy stimulates hormonal changes, which in turn, are associated with more nurturance toward babies. This perspective recognizes the dynamic or transactional nature of the links between hormones and behavior in which behavior changes can lead to hormonal shifts and vice versa. In contrast to the myth of the biologically unfit father, this work suggests that men may be more prepared even biologically for parenting than previously thought. Finally, it is critical to underscore that these hormonal changes are not necessary for the elicitation of fathering behaviors in either animals or humans (Fleming and Corter, 1995; Corter and Fleming, in Vol. 2 of this Handbook). In humans, for example, studies of father-infant relationships in the cases of adoption clearly suggest that hormonal shifts are unnecessary for the development of positive father-infant relationships (Brodzinsky, Lang, and Smith, 1995). Next is a discussion of the social determinants of father involvement.
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