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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 circmnstances 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. Keywords: child development; demographic transition; empathy; grandmother hypothesis; grandparental investment; grandparental solicitude; intergenerational transfers; kin altruism; maternal depression; reciprocal altruism doi: 10.1017/S0140525X09991105

Some commentators have argued that all particular altruistic acts are directly caused by or reinforced by an internal emotional state. Others argue that rewards obtained by one person might reinforce another person's altruistic act. Yet others argue that all altruistic acts are reinforced by social reciprocation. There are logical and empirical problems with all of these conceptions. The best explanation of altruistic acts is that--though they are themselves not reinforced (either immediately, or delayed, or conditionally, or internally)--they are, like self-controlled acts, part of a pattern of overt behavior that is either extrinsically reinforced or intrinsically reinforcing.

The capacity for flexible niche construction increases suddenly and dramatically when discrete memories and sensorimotor associations become woven into an interconnected worldview. Ontogenetic learning is as vital to culture as social learning because it is the wellspring of cultural novelty. Human altruism may result from pressures exerted at the level of cultural rather than biological replicators.

We propose a conceptual model that maps the causal pathways relating biological evolution to cultural change. It builds on conventional evolutionary theory by placing emphasis on the capacity of organisms to modify sources of natural selection in their environment (niche construction) and by broadening the evolutionary dynamic to incorporate ontogenetic and cultural processes. In this model, phenotypes have a much more active role in evolution than generally conceived. This sheds light on hominid evolution, on the evolution of culture, and on altruism and cooperation. Culture amplifies the capacity of human beings to modify sources of natural selection in their environments to the point where that capacity raises some new questions about the processes of human adaptation. Keywords: adaptation; altruism; cooperation; evolutionary psychology; gene-culture coevolution; human evolution; human genetics; niche construction; sociobiology

The target article authors' study has highlighted the relationship between market integration and an increased willingness to enter into cooperative exchanges. Less developed, albeit implied, in their analysis are the theoretical implications of their findings for the theory of altruism first developed by Adam Smith and later expanded in the works of the American historian, Thomas Haskell.

I argue that altruistic behavior and its variation across cultures may be caused by mental cognitive mechanisms that induce cooperative behavior in contract-like situations and adapt that behavior to the kinds of contracts that exist in one's socio-cultural environment. I thus present a cognitive alternative to Henrich et al.'s motivation-based account. Rather than behaving in ways that reveal preferences, subjects interpret the experiment in ways that cue their social heuristics. In order to distinguish the respective roles of preferences and cognitive processes that determine economic behavior, we need more ethnography of strategies 'in the wild.'

Group selection may be defined either broadly or narrowly. Narrowly defined group selection may involve either selection for altruism or group selection between alternative evolutionarily stable states. The last variety of group selection is likely to have been particularly important in human evolution.

Attention is drawn to significant distinctions implicit in our everyday nontechnical conceptual scheme. Three forms of motivation are distinguished: 'egoism,' 'altruism,' and 'groupism.' The relationship between them is considered. It is argued that groupish motives (as defined here) are not of themselves either altruistic or selfish ones in the relevant senses.

The data of Henrich et al., when combined with other research, suggest that sharing behavior probably varies systematically across cultures, situations, and individuals. Economic policies founded on recognition of this 'radical contingency' would, I argue, nurture economic pluralism rather than attempting to bring the world under one system.