What motivates grandparents to their altruism? We review answers from evolutionary theory, sociology, and economics. Sometimes in direct conflict with each other, these accounts of grandparental investment exist side-by-side, with little or no theoretical integration. They all account for some of the data, and none account for all of it. We call for a more comprehensive theoretical framework of grandparental investment that addresses its proximate and ultimate causes, and its variability due to lineage, values, norms, institutions (e.g., inheritance laws), and social welfare regimes. This framework needs to take into account that the demographic shift to low fecundity and mortality in economically developed countries has profoundly altered basic parameters of grandparental investment. We then turn to the possible impact of grandparental acts of altruism, and examine whether benefits of grandparental care in industrialized societies may manifest in terms of less tangible dimensions, such as the grandchildren's cognitive and verbal ability, mental health, and well-being. Although grandparents in industrialized societies continue to invest substantial amounts of time and money in their grandchildren, we find a paucity of studies investigating the influence that this investment has on grandchildren in low-risk family contexts. Under circumstances of duress – for example, teenage pregnancy or maternal depression – there is converging evidence that grandparents can provide support that helps to safeguard their children and grandchildren against adverse risks. We conclude by discussing the role that grandparents could play in what has been referred to as Europe's demographic suicide.

The effects of grandparental investment in relatives are apparent in human groups, suggesting that a postreproductive period in humans is selective. Although investment of relatives in kin produces obvious benefits for kin groups, selection for a postreproductive period in humans is not supported by evidence from chimpanzees. Instead, grandparental investment is likely a recent phenomenon of longevity, rather than an evolved feature.

Recent work on the evolution of utility has brought the blunt instrument of kin selection closer to the cluttered scalpel kit of social science. The concept of diminishing marginal utility can help streamline the latter. Reconciling ultimate causes with proximate inclinations, however, will be easier for the case of assistance from grandparents than assistance to them.

In order to evaluate the impact of contemporary grandparenting, the view should not be restricted to developmental achievement effects in grandchildren. Both child happiness and grandparent happiness are high-ranking goals with implications for public policy. The beneficial impact of grandparenting for risk families appears unequivocal, and modern life still encounters all kinds of unpredictable risks. Contemporary grandparental care is no evolutionary mismatch.

Grandparental altruism may be partially understood in the same way as other instances of altruism. Acts of altruism often occur in a context in which the actor has a broader sense of cause and effect than is evident in more typical behavioral interactions where cause and effect appear relatively transparent. Many believe that good deeds will ultimately produce good results.

Selection on grandparental investment is more complex than Coall & Hertwig (C&H) propose. Patterns of investment are subject to an intergenerational conflict over how resources should be distributed to maximize fitness. Grandparents may be selected to distribute resources unevenly, while their descendants will be selected to manipulate investment in their own favor. Here we outline the evolutionary basis of this conflict.

What makes the question of grandparental investment so very interesting is trying to tease out the underlying motivations. Grandparental investment is variable and grandparental altruism, if it exists at all, is also variable. Neither evolutionary theory nor rational choice theory has an easy time explaining this variation, and insight is further impeded by the absence of any compelling empirical studies designed for the purpose of testing alternative explanations of variations in grandparental investment.

Coall & Hertwig (C&H) bring attention to alternative accounts of grandparental investment from economics, evolutionary anthropology, psychology, and sociology, which have yet to be reconciled. We attempt to help integrate some of the disparate perspectives by expanding the scope of the evolutionary perspective, highlighting some gaps, and discussing problems with the authors' treatment of grandparents in traditional societies.

In the analysis of intergenerational transfer, several improvements can be made. First, following kin selection theory, grandparents have kin other than grandchildren in which to invest and therefore any investigation into grandparents should take this perspective. Secondly, how transfers actually enhance the survivorship of younger relatives such as grandchildren must be better measured, especially in the ethnographic literature. Finally, the problem of indirect investments or targeting must be considered.

The target article emphasizes the need to identify psychological mechanisms underlying grandparental investment, particularly in low-risk family contexts. We extend this approach by addressing the changing demands of balancing work and family in low-risk families. Taking a lifespan psychological perspective, we identify additional motivators and potential benefits of grandparental investment for grandparents themselves and for subsequent generations.

This commentary suggests that there is more continuity in pre- and post-demographic transition populations with respect to grandparental investments than is assumed by Coall & Hertwig (C&H). Recent research employing high-quality data supports the claim that sex-biased grandparental investments are likely to exist in industrialized societies, and that the economic status of grandparents is related to their long-term fitness.

Coall & Hertwig (C&H) suggest that, because grandparental investments do not impact on child mortality in low-risk societies, fitness effects are not plausible any more. We found that grandparental investments could very well alleviate contemporary constraints on fertility. Cultural factors may influence both the occurrence and impact of grandparental investments.

The question of why helpers help is debated in the cooperative breeding literature. Recent reevaluations of inclusive fitness theory have important implications for traditional populations in which the provisioning of young occurs in the context of intergenerational transfers. These transfers link older and younger generations in an economic relationship that both minimizes the demand for help and the cost of helping.

As societies industrialize, the age profile of consumption tilts strongly toward the elderly, while elder labor supply drops. Low fertility and long life lead to population aging. For millennia, material resources have, on net, flowed downward from older to younger within populations, but now in many rich societies net flows have reversed and go upwards from young to old.

To refine our understanding of grandparental investment and its consequences, we need to understand what grandparents do for their grandchildren. Knowing the landscape of grandparental investment will facilitate a better understanding of the impact of grandparental investment on grandchildren and will allow inroads to be made in bridging the different levels of analysis.

Coall & Hertwig (C&H) give an ambitious review about the broad range of grandparenting literature from the perspective of different disciplines. They aim to show, how evolutionary theory, sociology, and economics can mutually enrich each other. However, the differences between the evolutionary and the socio-economic perspective should be more clearly pointed out, because they usually deal with different research questions. Grandparents' well-being could be divided into its underlying components.

Coall & Hertwig (C&H) demonstrate the importance of grandparents to children, even in low fertility societies. We suggest policy-makers interested in reproductive timing in such contexts should be alerted to the practical applications of this cooperative breeding framework. The presence or absence of a supportive kin network could help explain why some women begin their reproductive careers “too early” or “too late.”

In discussing the effects of grandparents on child survival in natural fertility populations, Coall & Hertwig (C&H) rely extensively on the review by Sear and Mace (2008). We conducted a more detailed summary of the same literature and found that the evidence in favor of beneficial associations between grandparenting and child survival is generally weak or absent. The present state of the data on human alloparenting supports a more restricted use of the term “cooperative breeding.” Human stem family situations with celibate helpers-at-the-nest can be described as cooperatively breeding, but the term is a poor fit to many human family systems.

Integration of different lines of research concerning grandparental investment appears to be both promising and necessary. However, it must stop short when confronted with incommensurate arguments and hypotheses, either within or between disciplines. Further, some hypotheses have less plausibility and veridicality than others. This point is illustrated with results that conflict previous conclusions from evolutionary psychology about differential grandparental investment.

This response outlines more reasons why we need the integrative framework of grandparental investments and intergenerational transfers that we advocated in the target article. We discusses obstacles – from misconceptions to poor measures – that stand in the way of such a framework and of a better understanding of the effects of grandparenting in the developed world. We highlight new research directions that have emerged from the commentaries, and we end by discussing some of the things in our target article about which we may have been wrong.