We congratulate Coall & Hertwig (C&H) for their outstanding contribution. It certainly will have a noticeable impact on the respective research fields addressed therein. This work is a paramount example of the far too rarely occurring theoretical integration of work on one specific topic (here, grandparental investment) that is investigated across different disciplines and from distinct perspectives. This diversity of viewpoints is true for grandparental investment, which is studied in sociology, economics, as well as through the lenses of evolutionary theory, as applied in evolutionary psychology.
As C&H rightly conclude, significant parts of these literatures are distinct and at times disjointed and contradictory; are not well integrated; and have few discernible tendencies for fruitful exchange amongst themselves. Although we appreciate the general thrust of C&H's arguments, which calls for a broad integration of different viewpoints, we feel this might go too far and may well be “overly friendly” with respect to recent disparate views regarding the nature of grandparental investment. Simply put, opposing views and competing explanations of which one would rule out the opposite argument are unlikely to be equally plausible and hence both cannot be “true.”
This is, to some extent, reminiscent of past views (now outdated and quite clearly rebutted) in an entirely different research field, namely psychotherapy research. In the 1970s, this field became entrenched by opinions that now are encompassed by the “Dodo Bird Phenomenon” (alluding to the dodo figure appearing in Lewis Carroll's novel Alice's Adventures in Wonderland). Specifically, we now know that differential psychotherapy research (i.e., efficiency and efficacy studies comparing different psychotherapeutic schools, approaches, and techniques) erroneously concluded that, like in Carroll's novel, “everyone has won and all must have prizes” (cf. Luborsky et al. Reference Luborsky, Singer and Luborsky1975). These views have subsequently been quite clearly rebutted (e.g., Beutler Reference Beutler1991; Shadish & Sweeney Reference Shadish and Sweeney1991). With regard to opposing evidence about grandparental investment from the evolutionary, sociological, and economic literatures, there appears to be the potential danger of a similar “Dodo Bird Phenomenon.”
There is mounting evidence which suggests that important conclusions from the currently widespread evolutionary psychological reasoning about differential grandparental investment and solicitude might not be veridical, but rather be a result of neglected effects of confounding variables. For example, age and residential distance were found to be associated with investment and solicitude ratings, but were not statistically controlled in the studies of Euler and Weitzel (Reference Euler and Weitzel1996), Steinbach and Henke (Reference Steinbach and Henke1998), and Pashos (Reference Pashos2000). Specifically, evolutionary psychological research has introduced the notion that laterality effects are important in grandparental investment. There exist two distinct types of grandparents (matrilateral vs. patrilateral ones), which are distinguished by different degrees of paternity certainty, and thus, potentially different degrees of genetic relatedness (see target article, sect. 5.1.3.). Indeed, many studies conducted in modern industrialized nations (see target article, top of sect. 2. and Note 4) have obtained a specific pattern in the ranking of grandparental investment, as recalled by adults using rating scales, such that the maternal grandmother seemed to care the most, the maternal grandfather somewhat less, followed by the paternal grandmother, and then the paternal grandfather.
Elsewhere (Tran et al. Reference Tran, Fisher and Voracek2009), we show that the well-known spousal age differences, combined with the equally pervasive sex differences in life expectancy, create noticeable differences in the expected exposure time grandchildren have to their four grandparents. That is, due to the fact that grooms are generally older and become parents later in life than do brides, grandchildren have a shorter overlap in the number of years they could interact with grandfathers than they do with grandmothers.
It is important to note that the ranking of expected exposure time to the four grandparents follows exactly the above-mentioned ranking of recalled grandparental solicitude. Clearly, total grandparental investment in grandchildren must be limited by (and thus partly dependent on) the available time that grandparents have for such investments in their grandchildren. Intriguingly, appropriate controls for exposure time, accounting for the two confounding variables noted earlier, significantly level the matrilateral bias in grandparental investment (Tran et al. Reference Tran, Fisher and Voracek2009). We therefore conclude that the widely publicized matrilaterality effect of grandparental investment, as highlighted in current evolutionary psychological literature, may well be incorrect, or at least substantially overstated.
This finding has manifold ramifications and consequences. As for one example, some researchers (e.g., Gaulin et al. Reference Gaulin, McBurney and Brakeman-Wartell1997; Russell & Wells Reference Russell and Wells1987) have used such retrospective, obviously biased rating data of adults' recalled grandparental solicitude for estimating nonpaternity rates. Unsurprisingly, such calculations of putative nonpaternity rates have yielded grossly overstated and thus implausible values (10–20%) – which clearly indicates the inappropriateness of such rating data. In contrast, several recent systematic reviews of nonpaternity rates among modern human populations have convergently estimated these to be about 2–3% (Anderson Reference Anderson2006; Bellis et al. Reference Bellis, Hughes, Hughes and Ashton2005; Voracek et al. Reference Voracek, Haubner and Fisher2008).
Of further importance in this context, in their systematic review of grandparental investment and physical and mental well-being of grandchildren (sect. 5.1.), C&H cite studies (sect. 5.1.2.) that found differential grandparental investment (i.e., the matrilateral bias therein) to be dissociated from the criterion variables under scrutiny. This constitutes more counterevidence to current evolutionary psychological reasoning about the existence, importance, and consequences of differential grandparental investment, as the allegedly differentiated pattern of grandparental caring showed a lack of association with important life outcomes in the grandchildren.
Having taken evolutionary psychological study findings as examples for illustration, we therefore would suggest that the opposed views of sociology, economics, and evolutionary psychology on grandparental investment appear hard to reconcile and to integrate, as intended and anticipated by C&H. There may well be no “Dodo Bird Phenomenon” in these lines of research, and we do not believe that “everyone has won and all must have prizes.” At the very least, some of these competing hypotheses have less plausibility and veridicality than others. Specifically, we surmise that some important explanations and assertions in this field based on current evolutionary psychology thinking might be confounded or misleading, as they could be more parsimoniously derived from quite simple facts of demography, as we have elaborated here.
We congratulate Coall & Hertwig (C&H) for their outstanding contribution. It certainly will have a noticeable impact on the respective research fields addressed therein. This work is a paramount example of the far too rarely occurring theoretical integration of work on one specific topic (here, grandparental investment) that is investigated across different disciplines and from distinct perspectives. This diversity of viewpoints is true for grandparental investment, which is studied in sociology, economics, as well as through the lenses of evolutionary theory, as applied in evolutionary psychology.
As C&H rightly conclude, significant parts of these literatures are distinct and at times disjointed and contradictory; are not well integrated; and have few discernible tendencies for fruitful exchange amongst themselves. Although we appreciate the general thrust of C&H's arguments, which calls for a broad integration of different viewpoints, we feel this might go too far and may well be “overly friendly” with respect to recent disparate views regarding the nature of grandparental investment. Simply put, opposing views and competing explanations of which one would rule out the opposite argument are unlikely to be equally plausible and hence both cannot be “true.”
This is, to some extent, reminiscent of past views (now outdated and quite clearly rebutted) in an entirely different research field, namely psychotherapy research. In the 1970s, this field became entrenched by opinions that now are encompassed by the “Dodo Bird Phenomenon” (alluding to the dodo figure appearing in Lewis Carroll's novel Alice's Adventures in Wonderland). Specifically, we now know that differential psychotherapy research (i.e., efficiency and efficacy studies comparing different psychotherapeutic schools, approaches, and techniques) erroneously concluded that, like in Carroll's novel, “everyone has won and all must have prizes” (cf. Luborsky et al. Reference Luborsky, Singer and Luborsky1975). These views have subsequently been quite clearly rebutted (e.g., Beutler Reference Beutler1991; Shadish & Sweeney Reference Shadish and Sweeney1991). With regard to opposing evidence about grandparental investment from the evolutionary, sociological, and economic literatures, there appears to be the potential danger of a similar “Dodo Bird Phenomenon.”
There is mounting evidence which suggests that important conclusions from the currently widespread evolutionary psychological reasoning about differential grandparental investment and solicitude might not be veridical, but rather be a result of neglected effects of confounding variables. For example, age and residential distance were found to be associated with investment and solicitude ratings, but were not statistically controlled in the studies of Euler and Weitzel (Reference Euler and Weitzel1996), Steinbach and Henke (Reference Steinbach and Henke1998), and Pashos (Reference Pashos2000). Specifically, evolutionary psychological research has introduced the notion that laterality effects are important in grandparental investment. There exist two distinct types of grandparents (matrilateral vs. patrilateral ones), which are distinguished by different degrees of paternity certainty, and thus, potentially different degrees of genetic relatedness (see target article, sect. 5.1.3.). Indeed, many studies conducted in modern industrialized nations (see target article, top of sect. 2. and Note 4) have obtained a specific pattern in the ranking of grandparental investment, as recalled by adults using rating scales, such that the maternal grandmother seemed to care the most, the maternal grandfather somewhat less, followed by the paternal grandmother, and then the paternal grandfather.
Elsewhere (Tran et al. Reference Tran, Fisher and Voracek2009), we show that the well-known spousal age differences, combined with the equally pervasive sex differences in life expectancy, create noticeable differences in the expected exposure time grandchildren have to their four grandparents. That is, due to the fact that grooms are generally older and become parents later in life than do brides, grandchildren have a shorter overlap in the number of years they could interact with grandfathers than they do with grandmothers.
It is important to note that the ranking of expected exposure time to the four grandparents follows exactly the above-mentioned ranking of recalled grandparental solicitude. Clearly, total grandparental investment in grandchildren must be limited by (and thus partly dependent on) the available time that grandparents have for such investments in their grandchildren. Intriguingly, appropriate controls for exposure time, accounting for the two confounding variables noted earlier, significantly level the matrilateral bias in grandparental investment (Tran et al. Reference Tran, Fisher and Voracek2009). We therefore conclude that the widely publicized matrilaterality effect of grandparental investment, as highlighted in current evolutionary psychological literature, may well be incorrect, or at least substantially overstated.
This finding has manifold ramifications and consequences. As for one example, some researchers (e.g., Gaulin et al. Reference Gaulin, McBurney and Brakeman-Wartell1997; Russell & Wells Reference Russell and Wells1987) have used such retrospective, obviously biased rating data of adults' recalled grandparental solicitude for estimating nonpaternity rates. Unsurprisingly, such calculations of putative nonpaternity rates have yielded grossly overstated and thus implausible values (10–20%) – which clearly indicates the inappropriateness of such rating data. In contrast, several recent systematic reviews of nonpaternity rates among modern human populations have convergently estimated these to be about 2–3% (Anderson Reference Anderson2006; Bellis et al. Reference Bellis, Hughes, Hughes and Ashton2005; Voracek et al. Reference Voracek, Haubner and Fisher2008).
Of further importance in this context, in their systematic review of grandparental investment and physical and mental well-being of grandchildren (sect. 5.1.), C&H cite studies (sect. 5.1.2.) that found differential grandparental investment (i.e., the matrilateral bias therein) to be dissociated from the criterion variables under scrutiny. This constitutes more counterevidence to current evolutionary psychological reasoning about the existence, importance, and consequences of differential grandparental investment, as the allegedly differentiated pattern of grandparental caring showed a lack of association with important life outcomes in the grandchildren.
Having taken evolutionary psychological study findings as examples for illustration, we therefore would suggest that the opposed views of sociology, economics, and evolutionary psychology on grandparental investment appear hard to reconcile and to integrate, as intended and anticipated by C&H. There may well be no “Dodo Bird Phenomenon” in these lines of research, and we do not believe that “everyone has won and all must have prizes.” At the very least, some of these competing hypotheses have less plausibility and veridicality than others. Specifically, we surmise that some important explanations and assertions in this field based on current evolutionary psychology thinking might be confounded or misleading, as they could be more parsimoniously derived from quite simple facts of demography, as we have elaborated here.