At the end of their article, Coall & Hertwig (C&H) call for a comprehensive theory that takes into account evolutionary, economic, and sociological variables. Yet, even if there were such a theory, we might be no closer to a deeper appreciation of grandparents' behavior with respect to their children and grandchildren. Certainly, since the publication of our article (Friedman et al. Reference Friedman, Hechter and Kreager2008), in which we also reviewed the available empirical evidence, there have been no theoretically motivated empirical studies. To gather such data would be exceptionally demanding: A comparative test of the relative merits of evolutionary and rational choice (or exchange-theoretic) explanations would demand the study of investment behavior across a number of whole family sets wherein there were at least two children each of whom had at least one child. Furthermore, to be a proper test, the study would have to follow these family sets over time. Given the requirements for this research, we may be engaging in theoretical speculation for quite some time.
C&H advance the theoretical debate by their generous multidisciplinarity, but they muddy it by conflating dependent variables. Differential investment of time, money, and other resources in vivo is distinct from decisions about bequests, and so too is it distinct from opportunity for investment, which is yet different from its potential impact. Implications of grandparental solicitude for the public good in the form of potential policies are definitely premature. (If societies are planning to subsidize grandparental investment, is the outcome better if parents are subsidized for the care of their children rather than subsidizing grandparents for the care of their grandchildren? Or is it more efficient and effective to promote childcare as paid work?) There is a bit of a too-rosy picture of grandparents, as well: Grandparents get older, sick, die, exact an emotional price for their investments, and so on, even if they begin their solicitude pure in their motivations.
As we are collectively waiting for the researchers who will make the investments necessary to gather the data required for robust empirical tests of the major competing theories, there are some purely theoretical debates still to be had. For instance:
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(1) Suppose that the evolutionary model were, in fact, a faithful representation of grandparental motivation to invest? According to the model, we can count on parents to maximize the well-being of their children in order to ensure genetic fitness. Why, then, don't grandparents simply invest in their children and assume that their children will do the same, obviating the need for them to invest in their grandchildren, at least under low-risk conditions? Whereas evolutionary theorists may define parental investment as all the resources that parents invest in children to maximize their chance of survival to sexual maturity, it is well known that parents continue to invest massively in post-pubertal children, at least in developed societies (such as paying for their university tuition).
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(2) A fair amount is attributed to paternal uncertainty in evolutionary theory (contrary to our own predictions). If grandparents have the choice between investing in a son's children and in a daughter's children, what is the prediction? According to evolutionary theory, why would grandparents with a choice invest in a son's children at all? C&H note that “the general finding that maternal grandfathers provide more investment to grandchildren than paternal grandmothers … only held when the paternal grandmothers had other children in whom to invest” (section 3.6, para. 2). Indeed so: grandparental investment is variable across children.
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(3) How does empathy add to understanding of differential investment? If there is an inbred empathic capacity, how are we to explain variations in investment? Ditto for altruism.
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(4) Finally, the target article's Table 2 can only be taken seriously as evidence of any kind of grandparental impact on grandchildren if it is compared with the impact that any adult investment – related and unrelated adults – have on outcomes. Is a grandparent really better than a fabulous – and well-paid – nanny? The assumption that genetic fitness trumps pecuniary incentives is just that – an assumption, and a highly questionable one at that.
Recalling that our theory predicts that grandparents will invest in grandchildren of the child most likely to provide care for the grandparent toward the end of life, here are some hypotheses that we would want to add to our model, based on C&H's discussions:
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a. Grandparental investment will be indifferent to the age of the grandchild (see target article, sect. 3.4, para. 1).
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b. Grandparental investment is independent of grandparent-grandchild exchange and sensitive to grandparent-child exchange (sect. 4.1).
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c. Bequests are motivated by a different calculus than in vivo investments. Grandparents have no motivation to diverge from the equal division norm in death, based on our model.
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d. Grandparents who make a clear investment in one set of grandchildren are likely to have fewer depressive symptoms and reduced cognitive impairment (sect. 8.2, paras. 1–3).
All told, this is a topic that is ripe for a productive assessment of the relative merits of evolutionary and rational choice theories of individual behavior. The same is true for the much better appreciated phenomenon of declining fertility in developed societies. C&H advert to the demographic transition in their essay, but fail to point out that there is no satisfactory evolutionary explanation of it. Rational choice theory fares better in this respect (Friedman et al. Reference Friedman, Hechter and Kanazawa1994).
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At the end of their article, Coall & Hertwig (C&H) call for a comprehensive theory that takes into account evolutionary, economic, and sociological variables. Yet, even if there were such a theory, we might be no closer to a deeper appreciation of grandparents' behavior with respect to their children and grandchildren. Certainly, since the publication of our article (Friedman et al. Reference Friedman, Hechter and Kreager2008), in which we also reviewed the available empirical evidence, there have been no theoretically motivated empirical studies. To gather such data would be exceptionally demanding: A comparative test of the relative merits of evolutionary and rational choice (or exchange-theoretic) explanations would demand the study of investment behavior across a number of whole family sets wherein there were at least two children each of whom had at least one child. Furthermore, to be a proper test, the study would have to follow these family sets over time. Given the requirements for this research, we may be engaging in theoretical speculation for quite some time.
C&H advance the theoretical debate by their generous multidisciplinarity, but they muddy it by conflating dependent variables. Differential investment of time, money, and other resources in vivo is distinct from decisions about bequests, and so too is it distinct from opportunity for investment, which is yet different from its potential impact. Implications of grandparental solicitude for the public good in the form of potential policies are definitely premature. (If societies are planning to subsidize grandparental investment, is the outcome better if parents are subsidized for the care of their children rather than subsidizing grandparents for the care of their grandchildren? Or is it more efficient and effective to promote childcare as paid work?) There is a bit of a too-rosy picture of grandparents, as well: Grandparents get older, sick, die, exact an emotional price for their investments, and so on, even if they begin their solicitude pure in their motivations.
As we are collectively waiting for the researchers who will make the investments necessary to gather the data required for robust empirical tests of the major competing theories, there are some purely theoretical debates still to be had. For instance:
(1) Suppose that the evolutionary model were, in fact, a faithful representation of grandparental motivation to invest? According to the model, we can count on parents to maximize the well-being of their children in order to ensure genetic fitness. Why, then, don't grandparents simply invest in their children and assume that their children will do the same, obviating the need for them to invest in their grandchildren, at least under low-risk conditions? Whereas evolutionary theorists may define parental investment as all the resources that parents invest in children to maximize their chance of survival to sexual maturity, it is well known that parents continue to invest massively in post-pubertal children, at least in developed societies (such as paying for their university tuition).
(2) A fair amount is attributed to paternal uncertainty in evolutionary theory (contrary to our own predictions). If grandparents have the choice between investing in a son's children and in a daughter's children, what is the prediction? According to evolutionary theory, why would grandparents with a choice invest in a son's children at all? C&H note that “the general finding that maternal grandfathers provide more investment to grandchildren than paternal grandmothers … only held when the paternal grandmothers had other children in whom to invest” (section 3.6, para. 2). Indeed so: grandparental investment is variable across children.
(3) How does empathy add to understanding of differential investment? If there is an inbred empathic capacity, how are we to explain variations in investment? Ditto for altruism.
(4) Finally, the target article's Table 2 can only be taken seriously as evidence of any kind of grandparental impact on grandchildren if it is compared with the impact that any adult investment – related and unrelated adults – have on outcomes. Is a grandparent really better than a fabulous – and well-paid – nanny? The assumption that genetic fitness trumps pecuniary incentives is just that – an assumption, and a highly questionable one at that.
Recalling that our theory predicts that grandparents will invest in grandchildren of the child most likely to provide care for the grandparent toward the end of life, here are some hypotheses that we would want to add to our model, based on C&H's discussions:
a. Grandparental investment will be indifferent to the age of the grandchild (see target article, sect. 3.4, para. 1).
b. Grandparental investment is independent of grandparent-grandchild exchange and sensitive to grandparent-child exchange (sect. 4.1).
c. Bequests are motivated by a different calculus than in vivo investments. Grandparents have no motivation to diverge from the equal division norm in death, based on our model.
d. Grandparents who make a clear investment in one set of grandchildren are likely to have fewer depressive symptoms and reduced cognitive impairment (sect. 8.2, paras. 1–3).
All told, this is a topic that is ripe for a productive assessment of the relative merits of evolutionary and rational choice theories of individual behavior. The same is true for the much better appreciated phenomenon of declining fertility in developed societies. C&H advert to the demographic transition in their essay, but fail to point out that there is no satisfactory evolutionary explanation of it. Rational choice theory fares better in this respect (Friedman et al. Reference Friedman, Hechter and Kanazawa1994).
ACKNOWLEDGMENT
The authors are grateful for the comments of Satoshi Kanazawa.