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Does Mother Nature Punish
Rotten Kids?
Carl Bergstrom UW biology dept
Ted Bergstrom UCSB economics dept
Prelude: Economics as
Evolutionary Science
• Economists have been unwitting creationists
• Theory takes axiomatic approach to description of
human nature
• Axioms are typically quite arbitrarily selected
– Platonic rationality
– Behavioral this or that
• Standard defense of ideal rationality—How else to
find structure?
– Proposed answer: Evolution
Will Darwinian thinking change
economics?
• I am betting that it will– and that this will
engage economics much more deeply in the
other biological and social sciences.
Central Features of Evolutionary
Economics
• Evolution of Fundamental Preferences
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Selfishness vs altruism
Concern for relatives
Time Preference
Attitudes toward risk
• Evolution of rationality itself
• Evolution of institutions and beliefs—cultural
evolution
• Group selection versus individual selection
Evolutionary Economics of the
Family
• There is selection for behavior that maximizes
reproductive success.
– Broadly true whether transmission is genetic or cultural
• Marital institutions
• Inheritance and support of the elderly
• Fraternal cooperation, sibling rivalry, parentoffspring conflict
Theories of Parent-Offspring
Conflict in Biology
• Genetic-based conflict of interest—
Hamilton’s rule implies:
In sexually reproducing species, individuals
care half as much about their siblings as about
themselves.
Parents care equally about each of their children.
Parents quarrel with their children about
“Don’t be so selfish.”
The Parental Interest View
Biologist, Richard Alexander argues that
“Evolution selects for offspring that act to
maximize their parents’ reproductive
interest.”
Economist, Gary Becker’s “Rotten Kid
Theorem” reaches similar conclusion.
Alexander’s arguments
1) Animals who are overly greedy as
children, will suffer the evolutionary
penalty of having children that are overly
greedy.
Counter: (Sexual reproduction does not
produce identical copies.)
2) Parents are “bigger, stronger, smarter.”
Counter: Extortion
Parable of the Bleating Lamb:
A Cautionary Tale for Bad Children
The sheep model
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A Ewe lives for two years and has one lamb
each year.
She weans her lamb at some age x.
The first lamb’s own survival probability is an
increasing function of x.
The earlier she weans, the stronger the ewe will
be when she bears her second lamb, so the
second lamb’s survival probability is a
decreasing function of x.
Survival Probability Tradeoff
Lamb 2
M
L1
Lamb 1
Opponents and Allies
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Ewe wants to maximize sum of survival
probabilities.
Lamb 1 wants to maximize a weighted average
of own and Lamb 2’s survival probability, with
twice as big a weight for self.
Lamb 2 wants to maximize weighted average
with greater weight for self. But Lamb 2 is a
passive player in this game.
Mother loves firstborn, but their interests are
partly in conflict.
Finding preferred points
Lamb 2
L2
M
L1
Lamb 1
Genetic foundations of behavior
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Suppose that sheep have genes that tell them
what to do when they are lambs and other genes
that tell them what to do when they are mothers.
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Suppose (temporarily) that these genes are not
“linked”, so that simultaneous mutations in the
two behaviors are hard to maintain.
The Power of those who are
Weak but Loved by the Strong
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Lamb can not throw down its mother and force it
to nurse?
Ewe is faster, stronger, smarter.
What can lamb do?
What do small children do?
Bleat loudly.
Call the Wolf.
First-born’s preferred
Equilibrium
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Suppose that genes that command firstborn
lambs tell them to call the wolf unless they can
nurse to their preferred age.
In the same population, mothers are “soft” and
nurse the firstborn whenever it bleats.
This is an equilibrium.
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A mutant Mom who is hard-nosed will lose her
babies to the wolves. Her traits will not be passed on.
Mutant first-born who are less demanding will not
pass on their genes as often as the greedier first-born.
Mothers’ preferred Equilibrium
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Suppose that genes that command firstborn
lambs tell them to nurse when mother offers and
not to complain.
And the genes controlling maternal behavior tell
Mom to be hard-nosed. If lamb calls the wolf
when it is older than mom-optimal weaning age,
she ignores bleats and lets it take its chances with
the wolf.
This is an equilibrium.
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Mutant lamb who calls wolf is likely to be eaten and
less likely than normal lambs to pass on his genes.
Mutant mom who is less hard-nosed has fewer total
offspring.
Two possible equilibria
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Demanding lambs, compliant moms
Pliant lambs, Hard-nosed Moms
Both are evolutionary equilibria. The
second equilibrium is “more efficient” in
the sense that it reproduces more rapidly
``given the availability of resources.’’
Linked Genes
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Suppose that genes that control maternal
behavior and first-born behavior are closely
“linked”, so that if an animal gets two mutations,
one in each locus, these mutant genes are not
likely to separate in genetic recombination.
Then genetic combination, hard-nosed mom,
pliant lamb is likely to stick together and will
eventually outperform soft mom, demanding
lamb.
Cross-over illustrated
Soft Mom
Tough Mom
Demanding Kid
Pliant Kid
A Fatal combination (but nice for wolves)
Conclusion
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Whether the Becker-Alexander conjecture holds
true, depends on degree of genetic linkage
between genes for maternal behavior and those
for firstborn behavior.