Differential Investment Behavior between Grandparents and Grandchildren: The Role of Paternity Uncertainty

Evolutionary Psychology
www.epjournal.net – 2009. 7(1): 66-77
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Original Article
Differential Investment Behavior between Grandparents and Grandchildren:
The Role of Paternity Uncertainty
David I. Bishop, Department of Psychology, Luther College, Decorah, Iowa, USA. email:
bishopda@luther.edu (corresponding author).
Brian C. Meyer, Department of Psychology, Luther College, Decorah, Iowa, USA.
Tiffany M. Schmidt, Department of Psychology, Luther College, Decorah, Iowa, USA.
Benjamin R. Gray, Department of Psychology, Luther College, Decorah, Iowa, USA.
Abstract: Kin selection theory predicts that grandparents will differentially invest in their
grandchildren as a function of paternity certainty. This study explored the hypothesis of
“discriminative grandparental solicitude” (Euler and Weitzel, 1996; Smith, 1988) in a
sample of college students. Students with four living grandparents were asked to indicate
the frequency of various behaviors received from or directed to each grandparent. A
significant linear trend on a majority of the measures supported this hypothesis. Reported
contact and closeness were highest with the maternal grandmother (most genetically
certain) and lowest with the paternal grandfather (least genetically certain); maternal
grandfathers and paternal grandmothers were intermediate. The “preferential investment
hypothesis” (Laham, Gonsalkorale, and von Hippel, 2005) predicts that the investment
behavior of the maternal grandfather and the paternal grandmother should differ only when
there are cousins through the father’s sisters. Contrary to the predictions of this hypothesis,
grandchildren did not rate the maternal grandfather consistently higher on any of the
indices when more certain investment outlets were available to the paternal grandmother.
Keywords: discriminative grandparental solicitude, paternity uncertainty, preferential
investment.

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Grandparental solicitude
Introduction

In the human species parental certainty is asymmetrical. Human females can be
completely certain that their children are their own even if they do not know the identity of
the father. Because of concealed ovulation, internal fertilization, and risk of female
infidelity, however, human males can never be completely certain of their paternity. Thus,
the problem of paternity certainty is hypothesized to be an adaptive problem for human
fathers. According to kin selection theory, psychological adaptations may have evolved to
regulate investment in kin according to the degree of genetic certainty. These adaptations
are hypothesized to produce a differential in male and female investment psychology. The
logic of this psychology should extend to kin altruism of several types. Research on the
differential investment patterns of aunts and uncles (Gaulin, McBurney, and Brakeman-
Wartell, 1997); grandparents (Euler and Weitzel, 1996); and cousins (Jeon and Buss, 2007)
has been consistent with this prediction. The purpose of the present investigation is to test
predictions about grandparental solicitude using a sample of undergraduates who report
four living grandparents.

For human grandparents parental uncertainty may be present in two generations of
descendents. Grandfathers, for example, cannot be completely certain of the paternity of
their sons or daughters and paternal grandparents cannot be completely certain that their
son’s children are really their grandchildren. Maternal grandmothers, in contrast, are
completely certain that their daughters are really their own and that their daughter’s
children are really their grandchildren. Thus, for grandparents, genetic relatedness may
vary from completely certain to doubly uncertain. Again, kin selection theory predicts that
psychological adaptations may have evolved to regulate the investment in grandchildren as
a function of genetic certainty. Smith (1988) predicted that investment should vary as a
function of this certainty of relatedness: most for maternal grandmothers (MoMo; 0
uncertain links), least for paternal grandfathers (FaFa; 2 uncertain links), and intermediate
levels for maternal grandfathers (MoFa; 1 uncertain link) and paternal grandmothers
(FaMo; 1 uncertain link). Following Daly and Wilson’s (1980) “discriminative parental
solicitude,” Euler and Weitzel (1996) termed this differential investment “discriminative
grandparental solicitude.”

When grandparents are directly questioned about the discriminative care directed to
grandchildren their self-investment statements often suggest impartiality and equality of
care (Fischer, 1983; Thomas, 1989). Grandparents may not be inclined to declare, or may
not be aware of, differential favoritism (for exceptions to this tendency, see Michalski and
Shackelford, 2005; Pollet, Nettle, and Nelissen, 2006). Self-descriptive statements about
received solicitude, however, seem to be less influenced by such social desirability effects.
Consequently, research has typically focused on the recipients of discriminative care--the
grandchildren. Hoffman (1979-80) asked undergraduate females, with at least one living
grandparent, to report on the frequency of contact with, and their perceived closeness to,
each living grandparent. These grandchildren reported greater interactions with and being
emotionally closer to their maternal grandparents (one shared uncertain link) than paternal
grandparents (three possible uncertain links). Hartshorne and Manaster (1982) assessed the
amount of contact between adult grandchildren and each living grandparent at three time
periods: now, while in high school, and when home full time. Consistent with the
uncertainty hypothesis, grandchildren had the most contact with MoMo (no uncertain links)
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Grandparental solicitude
and the least contact with FaFa (2 possible uncertain links); contact with FaMo and MoFa
was intermediate (1 uncertain link). Euler and Weitzel (1996) had German adults rate (on a
seven-point scale) how much each biological grandparent had cared for them up until the
age of seven. Once again, the grandparent with the greatest degree of genetic certainty
(MoMo) invested the most, whereas the grandparent with the least degree of genetic
certainty (FaFa) invested the least. Salmon (1999) asked Canadian undergraduates to report
the frequencies of contact per annum (visits and phone calls) with their maternal and
paternal grandparents. These grandchildren reported more frequent contact with maternal
grandparents (one shared uncertain link) than paternal grandparents (three possible
uncertain links). In a study by Pashos (2000), Greek and German adults estimated (on a
seven-point scale) how much each grandparent had cared for them until the age of 7. Urban
Greek and German ratings were generally consistent with the ordinal rankings found in
prior studies. Finally, Laham, Gonsalkorale, and von Hippel (2005) replicated the
differential grandparental investment effect in a sample of Australian psychology students.
Respondents rated their emotional closeness to each of the biological grandparents they
could remember on a 0-100 “feelings thermometer.” As predicted, respondents reported
greater emotional closeness to their MoMo, followed by the MoFa, then the FaMo, and
finally the FaFa. While these studies are notable in their consistency in documenting the
solicitude effect, they share the significant limitation that not all grandparents were living at
the time of the self-report.

Recently, Laham, Gonsalkorale, and von Hippel (2005) have hypothesized that the
two grandparent types with one uncertain link each will show an investment differential
under certain circumstances. Specifically, MoFa will invest more in their grandchildren
relative to FaMo because some FaMo will have genetically more certain outlets available to
them. If cousins are available through the father’s sisters, a FaMo will have investment
outlets that are more certain than her investment outlet with her son. Consequently, she
may invest in her daughter’s children more heavily than her son’s children. In contrast, a
MoFa will not have a more certain outlet for his investment even if cousins are available
through the mother’s sisters. Thus, the “preferential investment hypothesis” predicts that
the preference for the MoFa should be significant only when there are cousins through the
father’s sisters. In a test of this hypothesis, respondents were asked to report how many
biological aunts and uncles they had on both sides of the family and how many biological
children each of these relatives have. As predicted by the preferential investment
hypothesis, MoFa were rated more highly than FaMo only when FaMo had grandchildren
via their daughters.

The purpose of the present investigation was to test the hypotheses of discriminative
grandparental solicitude and preferential investment using a sample of undergraduates who
report four living grandparents. Extant tests of grandparental solicitude have relied either
on retrospective reports of grandparental relationships from a given point in childhood or
on reports of current relationships with at least one living grandparent. Reports of the
former type return to a period in the respondent’s life when all grandparents are more likely
to be alive, and hence, differential solicitude can be assessed. This report strategy cannot
assess current investment behavior or current emotional closeness and may suffer from
retrospective distortion (e.g., second-hand descriptions of grandparents). Reports of the
latter type allow the assessment of current investment behavior and emotional closeness but
suffer from differential grandparent mortality, making assessment of differential solicitude
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Grandparental solicitude
questionable. In one study, for example, approximately 60% of MoMo were still living at
the time of assessment but only 30% of FaFa were still alive (Hoffman, 1979-1980). While
some degree of differential investment should be evident despite differential mortality, an
unconfounded test of this effect requires living grandparents in each category. To date, no
study has tested the differential solicitude effect while insisting on four living grandparents.
By restricting our sample to grandchildren with four living biological grandparents, we
could assess differential investment and preferential investment by directly examining the
two-way exchange of various investment behaviors between grandchild and grandparent
type. In addition to a “feelings thermometer” rating, we asked respondents to indicate the
number of telephone calls made, letters written, gifts given, etc. These behaviors should
also reflect the closeness of the grandchild and grandparent dyad and may be less prone to
response bias and/or retrospective distortion. We tested the following predictions: (a)
reported investment behavior and rated closeness between grandchildren and grandparents
will vary as a function of the number of uncertain links between grandparents and
grandchildren and (b) the investment of MoFa will be greater than the investment of FaMo
only when there are cousins through the father’s sisters.
Materials and Methods
Respondents

One hundred and forty students (89 women and 51 men; mean age ± SD = 19.49 ±
1.15) from a Midwestern liberal arts college were approached opportunistically at their
place of residence and asked to complete a series of questionnaires about their relatives.
Only students who reported having four living biological grandparents were asked to
complete the questionnaires. Upon completion of the questionnaires the students received a
redeemable delicatessen coupon.

Procedure

The questionnaire packet consisted of a participant data sheet and four
questionnaires. The data sheet required that respondents list the names of their grandparents
by relationship (e.g., Paternal grandfather’s name (your father’s father): list name here) and
their respective place of residence. The data sheet also required that respondents list
paternal and maternal aunts and uncles and their respective number of biological children.

Following the completion of the data sheet, respondents were asked to complete a
one page questionnaire about each of their four living grandparents. Each questionnaire
consisted of 12 questions (see Table 1) pertaining to the grandparent listed at the top of the
page. Eleven of the questions asked the respondent to report on the frequency of a behavior
directed to a grandparent (over the last year) or the frequency of a behavior received from a
grandparent (over the last year). One question asked the respondent to report their degree of
emotional closeness (on a 0-100 scale) to the grandparent in question. The presentation of
the grandparent questionnaires within a packet was counterbalanced.


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Grandparental solicitude
Results

Transformation of raw scores

A preliminary screen of the distributions of the raw scores from each question
revealed significant skew. Questions 1 through 11 were positively skewed and question 12
was negatively skewed. Following transformation procedures suggested by Tabachnick and
Fidell (2001), raw scores were transformed to achieve closer approximations to normality.
A value of 1 was added to each score on questions 1 through 11 to remove zero as a value.
Questions 1 through 11 were then re-expressed using a logarithmic transformation. On
question 12 each score was subtracted from a constant (largest possible value + 1) and then
re-expressed using a logarithmic transformation. These transformations achieved closer
approximations to normality.

Differential solicitude by number of uncertain links

Means and standard errors for each grandparent by question (before transformation)
are presented in Figure 1. A series of repeated-measures ANOVAs (on transformed scores)
confirmed that investment behavior and rated closeness between grandchildren and
grandparents varied as a function of the number of uncertain links (see Table 1 for
transformed means and standard errors). There was a significant effect of grandparent type
on all 12 questions. A significant linear trend was confirmed for 10 of these 12 measures.
As predicted, grandchildren reported the greatest contact and the highest level of rated
closeness with MoMo (0 uncertain links) and lowest level of contact and the lowest rated
closeness with FaFa (2 uncertain links). As predicted, MoFa (1 uncertain link) and FaMo
(1 uncertain link) were intermediate on all of the measures. Although MoFa were rated
higher on some measures and FaMo on others, planned comparisons of adjacent group
means showed that MoFa and FaMo were significantly different on only question 6
(F(1,138) = 7.49, p < .01). When the same analyses were performed on the data before
transformation, there was a significant effect for grandparent type on only 9 of the 12
questions. The presence of a significant linear trend was confirmed for 8 of the 9 measures.

Distance between each grandparent and reporting grandchild

The residential distance between grandparent and grandchild has been considered a
potential confounding variable by previous researchers (Euler and Wietzel, 1996; Pashos,
2000). For example, the distance between grandparent and grandchild might affect the
frequency of visits to and from grandparents. To assess the influence of this potential
confound, we calculated the distance in miles between the residential cities of the
respondent and each of their four grandparents using the web-based Geobytes City
Distance Tool. This tool calculates distance “as the crow flies” using the longitude and
latitude of each city. The four distances were then entered as covariates into a series of
one-way repeated-measures ANCOVAs (on transformed scores). Distance as a covariate
was significant only on items 7 (visited grandparent), 8 (grandparent visited), and 11 (give
a gift). Nevertheless, the covariate-adjusted means for these three items were virtually
identical to the means reported in Table 1 above. Consequently, distance does not seem to
affect the patterns of differential solicitude assessed in this study.

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Grandparental solicitude
Figure 1. Means (+ S.E.) for each grandparent by question (before transformation).

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Preferential investment hypothesis

The preferential investment hypothesis predicts that the investment of MoFa should be
greater than the investment of FaMo only when there are more certain genetic outlets for
the FaMo--namely, cousins through the father’s sisters. A series of 2 (MoFa vs. FaMo) by
2 (no cousins vs. cousins) repeated-measures ANOVAs was performed on each investment
question. Transformed means and standard errors for MoFa and FaMo as a function of
cousins by the father’s sisters and interaction F statistics are presented in Table 2. The
predicted interaction for question 1 (called grandparent) was significant. Contrary to
predictions, however, simple effects analysis revealed that MoFa scores were not
significantly higher than FaMo scores when FaMo have grandchildren via their daughters,
t102 = -1.60, p > .05. The predicted interaction for question 2 (grandparent called) was
significant. Contrary to predictions, simple effects analysis revealed that FaMo scores were
significantly higher than MoFa scores when FaMo have grandchildren via their daughters,
t102 = -2.60, p < .05. The predicted interaction for question 7 (visited grandparent) was
significant. Simple effects analysis revealed that MoFa scores were not significantly
different from FaMo scores when FaMo have grandchildren via their daughters, t102 = .02,
p > .05. The predicted interaction for question 8 (grandparent visited) was significant.
Once again, simple effects analysis revealed that MoFa scores were not significantly
different from FaMo scores when FaMo have grandchildren via their daughters, t102 =
-1.48, p > .05. The predicted interaction for question 10 (received gift) was significant.
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Consistent with prior analyses, the simple effects analysis revealed that MoFa scores were
not significantly different from FaMo scores when FaMo have grandchildren via their
daughters, t 102 = -.84, p > .05. Finally, the predicted interaction for question 12 (rated
closeness) was significant. Contrary to predictions, simple effects analysis revealed that
MoFa scores were not significantly different from FaMo scores when FaMo have
grandchildren via their daughters, t102 = .91, p > .05. The predicted interactions for
questions 3, 4, 5, 6, 9, and 11 were not significant. A parallel set of analyses was conducted
on the data before transformation. Without the transformation, the predicted interactions for
questions 3, 4, 7, 9, 10, 11, and 12 were not significant. On the questions yielding a
significant interaction, simple effects analysis revealed that MoFa scores were not
significantly higher than FaMo scores when FaMo have grandchildren via their daughters.


Discussion

The present study replicates and extends the work on discriminative grandparental
solicitude by assessing multiple acts of investment behavior and emotional closeness with
four living grandparents. By restricting our sample of grandchildren to those who report
four living grandparents, we could ask respondents to report actual behaviors received from
or directed to living grandparents. In addition to these direct behavioral measures, we also
employed the single-item rating of emotional closeness used in prior investigations.
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Grandparental solicitude
Consistent with the predictions of Smith (1988), there was a significant inverse linear trend
between reported acts of solicitude and degree of parental uncertainty. Grandchildren
reported the greatest contact and the highest level of rated closeness with MoMo (0
uncertain links) and lowest level of contact and the lowest rated closeness with FaFa (2
uncertain links). As expected, MoFa (1 uncertain link) and FaMo (1 uncertain link) were
rated as intermediate and were not reliably different. Moreover, the discriminative
solicitude effect was present on both behaviors received from grandparents and directed to
grandparents. MoMo were more likely to write, give gifts, visit, send money, and call than
FaFa. MoMo were also more likely to receive a gift, receive a call, and receive a visit than
the FaFa. Our findings are consistent with prior investigations based on retrospective
reports from childhood and contemporaneous reports of investment behavior with a subset
of living grandparents. To our knowledge, this is the first study testing differential
grandparental investment that has required all four grandparents to be alive at the time of
the assessment.
Differential
grandparental
solicitude has also been studied by decomposing the four
grandparent types into their two constituent components; namely, sex of grandparent and
side of family. This approach to grandparental altruism treats sex of grandparent and side
of family as independent effects and implies different underlying causal mechanisms for
each effect. A number of researchers have approached differential grandparental
investment from this perspective and reported both main effects and interactions (Hoffman,
1979-1980; Pashos, 2000; Salmon, 1999). While Pashos argues that cultural variables will
affect sex of grandparent and side of family (as independent effects) differently, the
presence of the reliable linear relationship between grandparent types requires that
statements about main effects be carefully qualified. For example, as Euler and Weitzel
(1996) correctly note, grandmotherly solicitude does not apply to both grandmothers
equally but is more pronounced in the maternal grandmother. As our study confirms, the
differentiation of all grandparent types yields the predicted linear function of solicitude on
most of the variables we examined. Jeon and Buss (2007) have recently extended this same
logic to the study of differential altruism among cousins.

We also tested the preferential investment hypothesis using multiple measures of
investment and closeness with four living grandparents. Laham, Gonsalkorale, and von
Hippel (2005) proposed that FaMo will invest less in their son’s grandchildren when they
also have grandchildren through their daughters. Under these circumstances, MoFa should
make a greater investment in their grandchildren relative to FaMo. Contrary to the
predictions of the preferential investment hypothesis, grandchildren did not rate the MoFa
consistently higher on the direct behavioral indices or on closeness when more certain
investment outlets were available to the FaMo. For grandparents who each have one
uncertain link, the presence of more certain investment outlets does not seem to affect the
calculus of their investment. While these findings call into question the preferential
investment hypothesis, the findings are nevertheless consistent with the hypothesis of
differential grandparental solicitude.

In the human species parental certainty is asymmetrical; consequently, both parental
and grandparental solicitude should be discriminating. On both direct measures of
investment and rated closeness with four living grandparents, we found that grandparental
solicitude varied predictably as a function of parental certainty. Although we found no
support for the preferential investment hypothesis when all four grandparents were alive,
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