replication, the picture is quite different. Biologist William Hamil-
ton combined empirical data with mathematical models to show
under what circumstances the individual's self-sacrifice would propa-
gate the genes that favor that behavior. When all workers in a colony
are sisters and the queen's offspring share genes with them, when
they sacrifice themselves they help propagate genes that they share.
Now humans too display some dispositions that can be explained in
terms of kin selection. We not only spend resources and energy for
our offspring's benefit but we also cooperate with kin much more
readily than with unrelated persons, and on terms that are quite dif-
ferent.^10 [183]
Still, kin selection is obviously not the only factor since humans
from times immemorial have engaged in cooperation with unrelated
(or very remotely related) individuals. But this, too, is found in other
animal species besides humans, and constitutes reciprocal altruism, fol-
lowing the principle "I scratch your back and you scratch mine."
Familiar examples include blood-sharing among vampire bats. Biolo-
gist Robert Trivers showed how reciprocating strategies could evolve
under specific conditions of population, reproduction and use of
resources, but also when specific capacities are present in individuals.
Cooperation requires that animals can discriminate between loyal
partners, who tend to reciprocate a good turn, and freeloaders, who
do not. Apparently indiscriminate sharing is in fact judiciously mea-
sured. This seems to be the case in bats or dolphins or other species
with complicated brains, and of course in humans who do preserve
memories of past episodes of interaction with other humans. Again,
although such strategies are individually unselfish they remain genet-
ically selfish. The genes that carry them stand a good chance to prop-
agate even when other strategies appear in the local gene pool.^11
Incidentally, what is called a "strategy" in these accounts is of
course not based on conscious deliberation. The bats that exchange
blood do not reflect on the possible consequences of sharing or not
sharing. A strategy is just an organized way of behaving. This is the
same as "deciding" how to stay upright. You do not have to think
about it, but a special system in the brain takes into account your cur-
rent posture, the pressure on each foot, and corrects your position to
avoid a fall. In the same way, these accounts show that specialized
cognitive systems register situations of exchange, store them in mem-
ory and produce inferences for subsequent behavior, none of which
requires an explicit consideration of the various options available.
WHYDOGODS ANDSPIRITSMATTER?