1. Parental (and Grandparental) Investment
The important concept of parental investment.
- Some psychological implications.
Sex differences in parental caregiving.
- Underlying logic: obligatory parental investment & paternity uncertainty
Additional implications of paternity uncertainty.
- Including implications for grandparental care-giving.
Additional research on grandparental care-giving.
- The “grandmother hypothesis.”

2. Dawkins, Chapters 7 and 8:
Among other things:
- “Bearing” and “caring”
- Parental investment and its implications
- Parent-offspring conflict
- “Grandchild altruism”

3. Why “caring” (and not just “bearing”) matters…

4. a “problem” ! One representation of typical primate “life histories”
Humans must live for many years before reaching sexual maturity.
Humans spend several years being pretty darn helpless.

5. One solution to that problem: Parental investment.
“…any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.”
(Trivers, 1972)
Many forms of parental investment:
Protection of offspring from harm.
Provision of resources to offspring.
Nurturant behaviors of many kinds directed toward offspring.
Offspring benefit from parental care.
Offspring benefit from biparental care.
Psychological adaptations facilitating
parental care behavior?

6. Parental investment and “adaptive memory”:
Enhanced memory for parenting-relevant things.
(Seitz, Polack, & Miller, 2018)
Procedure: List of words, rated for relevance to an imagined situation.
Measure: Percentage of words recalled later, depending on imagined situation.
Survival: …imagine that you are stranded, thousands of years ago, in the grasslands of a foreign land, without any basic survival materials. Over the next few months, you’ll need to find steady supplies of food and water and protect yourself from predators.
Monument building: …imagine that you are living, thousands of years ago, in the grasslands of a foreign land, and are building a spiritual monument. Over the next few months, you’ll need to collect rocks and other materials to complete this monument.
Mating: …imagine that you are living, thousands of years ago, in the grasslands of a foreign land, and are looking to find a mate. Over the next few months, you’ll need to find potential mates who would be best for producing the most numerous and healthy offspring.
Parenting: …imagine that you are living, thousands of years ago, in the grasslands of a foreign land, and have just parented a baby. Over the next few months, you’ll need to provide care and nourishment for your baby and help him/her grow.

7. Parental investment and “adaptive memory”:
Enhanced memory for parenting-relevant things.
(Seitz, Polack, & Miller, 2018)
Survival
Monument
Mating
Parenting

8. Parental investment.
“…any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.”
(Trivers, 1972)
Some costs associated with parental investment:
Reduced resources to provide to other offspring.
Reduced opportunities to make more offspring.
“mating / parenting trade-off.”
Implications for:
Parent / offspring conflict.
Sex differences in parental care-giving.
Sex differences in mating attitudes and behaviors.

9. Sex differences in parental care-giving. Evolutionary bases:
Sex differences in obligatory parental investment.
Sex differences in certainty of maternity / paternity.
Straightforward implication:
Sex differences in parental attitudes toward children.
Additional implications of paternity uncertainty:
Sexual jealousy and “mate guarding.”
Reassurance from mothers and other relatives.
In spontaneous statements at time of child’s birth:
Mothers were 4 times more likely than fathers to say anything about who the child looked like.
Mothers were 4 times more likely to say that the child looked like the father than to say that it looked like themselves.
(Daly & Wilson, 1982)

10. Sex differences in parental care-giving. Evolutionary bases:
Sex differences in obligatory parental investment.
Sex differences in certainty of maternity / paternity.
Straightforward implication:
Sex differences in parental attitudes toward children.
Additional implications of paternity uncertainty:
Sexual jealousy and “mate guarding.”
Reassurance from mothers and other relatives.
Grandparental care.

11. Grandparental care.
(How much care do specific grandparents provide to their grandchildren?)
(And why?)

12. Implications of paternity uncertainty…
Father’s
Father
Father’s
Mother
Mother’s
Father
Mother’sMother
Father
Mother
Child
Implications of paternity uncertainty…

13. ? ? ? ¢ $$$ Father’s Father Father’s Mother Mother’s Father
Mother’sMother ? ?
Father
Mother ?
Child
Least certainty about kinship =
Least grandparental investment
Greatest certainty about kinship =
Greatest grandparental investment
$$$

14. ? ? ? ¢ $$$ Father’s Father Father’s Mother Mother’s Father
Mother’sMother ? ?
Father
Mother ?
Child
Least certainty about kinship =
Least grandparental investment
Greatest certainty about kinship =
Greatest grandparental investment
$$$

15. ? ? Father’s Mother Father’s Sister Father’s Sister Father’s Brother
Cousins
($$)
Cousins
($$)
Cousins
($)
Child
($)
Some paternity uncertainty;
tied for least certain (compared to other grandchildren)

16. ? ? ? ? ? ? Mother’s Father Mother’s Sister Mother’s Brother Mother’s
Child
($$)
Cousins
($$)
Cousins
($)
Cousins
($)
Some paternity uncertainty;
tied for most certain (compared to other grandchildren)

17. Results of many studies:
Father’s
Father
Father’s
Mother
Mother’s
Father
Mother’sMother $ $$
$$$
Results of many studies:
Grandparents’ investment of resources in their grandchildren follows the pattern predicted by the logical implications of paternity uncertainty.

18. One more thing on grandparental investment:
The “grandmother hypothesis”...

19. typical primate “life histories”
One representation of
typical primate “life histories”
Female humans have a long “post-reproductive” lifespan.
Humans must live for many years before reaching sexual maturity.
Humans spend several years being pretty darn helpless.

20. The “grandmother hypothesis”:
Evolutionary explanation for female menopause and human longevity.
“As long as postmenopausal women contribute to the welfare of their kin, they affect the successful reproduction of their genes. [It has been] hypothesized that menopause evolved when other changes in our lineage made late births riskier and infants more dependent. Older mothers would be more likely to die in childbirth, leaving orphans unable to survive without them… these circumstances would favor tendencies to stop fertility early and for mothers to invest more in previously born offspring than in risky new ones.”
(Hawkes & Coxworth, 2013)
(…and in offspring’s offspring!)

21. Research on the benefits of (post-menopausal) grandmothers:
Hadza hunter-gatherers in Tanzania:
Grandmothers essential in foraging for food for families.
Villages in rural Gambia:
Grandchildren more well-nourished if they had a living post-menopausal (compared to pre-menopausal) grandmother.
Historical data from Canada and Finland:
Grandchildren more likely to survive to adulthood if they had a living post-menopausal grandmother.

22. New research on menopause and grandparental care:
(Hofer et al., unpublished)
Two big datasets in two different countries (U.S., Australia).
Comparisons of pre-menopausal and post-menopausal grandmothers.
Measure of time spent caring for grandchildren.
Measure of a different form of altruistic behavior (volunteering).
Compared to pre-menopausal grandmothers, post-menopausal grandmothers spend more time caring for grandchildren.
But weren’t more altruistic in general.