Coall & Hertwig (C&H) call upon researchers to integrate their perspectives to advance our understanding of grandparental investment and its impact on the lives of grandchildren. Clarifying what we mean by grandparental investment may be a worthy step in this endeavor. A complete understanding of the nature of grandparental investment and the effects of grandparental investment on grandchildren will require a comprehensive understanding of what such investment is. C&H cite numerous studies looking at the impact of grandparental investment on various outcomes for grandchildren. Most of these studies use measures of grandparental investment that are limited in scope and were likely valued because they were economical to include in large, representative studies. Such measures have outlived their utility. Social distance, time/contact frequency, money, care, and affection represent a sampling of the variables used in the studies cited by C&H. Grandparental investment, of course, can take many forms, which are highlighted in the target article, but this list is not comprehensive and is inadequate. I will single out one variable.

Spending time with grandchildren can be a useful variable in some studies, but prospecting the behaviors that grandparents engage in while spending time with their grandchildren may prove more valuable in identifying the evolved psychological mechanisms producing these investments. Informing grandchildren about kin relationships and ancestors, offering advice, settling disputes, providing encouragement, showing interest in the activities of grandchildren are just a few of the things that grandparents can do when spending time with grandchildren (Euler & Michalski Reference Euler, Michalski, Salmon and Shackelford2007). These potentially revealing measures are masked when we use contact frequency as a sole measure of grandparental investment. Time spent with grandchildren in a given period of time is a domain-general measure. Posing domain-general questions thwarts our ability to identify the domain-specific operation of evolved psychological mechanisms adapted for channeling investment to grandchildren. The use of such domain-general measures may be contributing to the difficulty researchers have in crossing interdisciplinary boundaries in order to advance our understanding of grandparental motivations. Contact frequency does not necessarily lend itself to a thorough integration of theoretical perspectives, nor does it inform us about what it is about contact frequency that may be associated with developmental outcomes.

C&H note that there is little to no convincing evidence of sex-biased grandparental investment in humans. This statement may reflect the inadequacy of extant measures of grandparental investment. Grandparents may, for example, channel different investments toward granddaughters than to grandsons, but these potential variables have not yet been empirically documented because of limitations in our tools for assessing grandparental investment. Examining the different types of grandparental investments will allow for more thorough examinations of economic, sociological, anthropological, psychological, and biological discussion.

We also need to exercise care in interpreting reports of grandparental investment. Grandparental, parental, and grandchild reports of grandparental investment may reveal variation attributable to reporting biases. Appreciating the biases that may be prevalent in reports of grandparental investment will help us to better understand how grandparental investment is perceived by kin. In understanding which grandparents invest more than others, for example, Euler et al. (Reference Euler, Hoier and Rohde2001) note that responses provided by the recipients of investment may be more accurate, because grandparents may deceive themselves into believing that they do not favor one or a few of their grandchildren.

C&H note the empirical studies documenting biases of grandparents toward grandchildren related through daughters than through sons and related through firstborns and lastborns than through middleborns. This evidence hints at the evolved psychology of parents in mediating grandparental investment and the evolved psychology of grandparents by their responsiveness to parents. A goal of researchers studying grandparental investment might be to better understand the role of parents in encouraging (or discouraging) grandparental investment. What do parents do to encourage grandparents to invest in their grandchildren? Does this encouragement come at the expense of grandparents' investment budgets for grandchildren related through siblings of the parent? To my knowledge, there exists no study examining the ways in which parents attempt to encourage their parents to invest in grandchildren.

The type of investment that grandparents make should matter in the developmental outcomes of grandchildren, and may jump-start the integration of different theoretical approaches to the study of grandparental investment. Understanding the nature of grandparental investment by means of knowledge of the evolved psychological mechanisms activated in the presence of grandparents, parents, and grandchildren will amplify research efforts and further inform public policy decisions.