Via Barker, a pointer to a theory from evolutionary psychology that tears are a true signal that the person crying is vulnerable and in need.
Emotional tears are more likely, however, to function as handicaps. By blurring vision, they handicap aggressive or defensive actions, and may function as reliable signals of appeasement, need or attachment.
Usually you should be skeptical that signaling is evolutionarily stable. For example if tears convince another that you are defenseless then there is an evolutionary incentive to manipulate the signal. Convince someone you are defenseless and then take advantage of them.
A typical exception is when the signal is primarily directed toward a family member. Family members have common interests because they share genes. Less incentive to manipulate the signal means that the signal has a better chance of being stable. And babies of course have few other ways of communicating needs.
Of course children eventually do start manipulating the signal. They learn before their parents do that they are becoming self-sufficient but they still have an incentive to free-ride on the parents’ care. Fake tears appear. But this is a temporary phase until the parents figure it out. Not surprisingly, once the child reaches adulthood, crying mostly stops: Nature takes away a still-costly but now-useless signal.
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April 29, 2010 at 8:40 pm
涙の効用 » 経済学101
[…] Credible Tears By blurring vision, they handicap aggressive or defensive actions, and may function as reliable signals of appeasement, need or attachment. […]
April 30, 2010 at 10:20 pm
Robert Wiblin
Why is it a strategy (for your genes) to be inclined to fool your parents into giving you extra resources once you’re a bit older, but not a good strategy for them to make you trick them instinctively with lots of crying when you’re a very young baby?
May 1, 2010 at 2:08 am
Ryan
This made me think of something interesting. What kind of evolutionary balance is there between parents and their offspring? Since parents ultimately control the set of genes available to their offspring (i.e. it is determined by their own genes), is there an evolutionary incentive to have offspring that are less costly for the parents?
One possible example is the size of humans heads. Even though brain power could be argued as an important characteristic for success, why don’t our heads get any bigger? One possible answer is childbirth. Too big of a head makes childbirth impossible. Hence head size is constrained by mothers’ hip size. Yet it’s not clear that this is selection by the child or the parent, because a child with too large of a head at the time of birth not only risks the health of the mother (which also effects his/her ability to be raised) but also risks his/her own life at birth.
If there was a gene that risked the health of the mother, yet still provided a net benefit to the child (such as head size), I would expect the gene to be carried on the Y chromosome, because such a gene would be evolutionarily costly on the mother who evolutionarily hopes to have more children. This is why I think boys are more difficult to raise than girls. Actually, I just now looked it up online and confirmed that “on average a male’s head is 1.33 cm larger than that of a female” (median age of sample was 30 years). So this fact seems to fit my hypothesis, but it would really matter the size at birth (still, there is good reason to believe that adult head size is correlated with infant head size).
If it were possible for the mother to somehow select which genes she passes on, then I perhaps this problem would take more thought.