Tag Archives: natural selection

Of Crackers and Quackers: Human-Duck Social Interaction is Regulated by Indirect Reciprocity (A Satire)

1280px-221_Mallard_DuckWatching the ducks on a neighborhood pond can be an entertaining and rewarding pastime. I myself, along with my nine-year-old co-investigator, have taken daily opportunities to feed some ducks on a nearby pond over the past several months. In doing so, we not only had fun but also managed to conduct some urban science that led us to a new scientific discovery: Mallards (Anas platyrhynchos L.) engage in indirect reciprocity with humans. Scientists have known for decades, of course, that indirect reciprocity was critical to the evolution of human social interaction in large-scale societies, but we believe we are the first to identify indirect reciprocity at work in human-duck social interaction.

Here’s how we made this discovery.

On random days, we take a soda cracker along with us to feed to a single lucky duck. On the other days, we take our walks without a cracker. What my young co-investigator and I have noticed is that on cracker days, after we’ve fed the cracker to the first duck that approaches us (the “focal duck,” which we also call “the recipient”), other ducks (which we call “entertainment ducks,” or “indirect reciprocators”) appear to take notice of our generosity toward the recipient. Almost immediately, the indirect reciprocators start to perform all sorts of entertaining behaviors: They swim toward us eagerly, they waddle up to us enthusiastically, they stare at us with their dead, obsidian eyes, they quack imploringly. It’s all very amusing and my co-investigator and I have a great time. Take note of the fact that we always bring only a single cracker with us on cracker days. As a result, the indirect reciprocators have absolutely nothing to gain from the entertainment they provide. In fact, they actually incur costs (in the form of energy expended and lost foraging time) when they do so. Thus, their indirect reciprocity behavior is altruistic.

Our experience with the indirect reciprocators is very different on non-cracker days. If a focal duck comes up to us on a non-cracker day, there’s just no cracker to be had, no matter how charming or insistent the request. Dejected, the focal duck typically waddles or paddles away within a few seconds. Now, what do you suppose the entertainment ducks do after we refuse to feed the focal duck? That’s right. They withhold their entertainment behaviors. This pattern, of course, is exactly as one would expect if the entertainment ducks were regulating their entertainment behaviors according to the logic of indirect reciprocity.

Theorists typically assume that the computational demands for indirect reciprocity to evolve are quite extensive. For instance, indirect reciprocators need to possess computational machinery that enables them to acquire information about the actions of donors—either through direct sensory experience of donor-recipient interactions, or (more rarely) language-based gossip, or (even more rarely) social information stored in an external medium, such written records or the reputational information that’s often available in online markets. Indirect reciprocators also need be able to tag donors’ actions toward recipients as either “beneficial” or “non-beneficial,” store that social information in memory, and then feed that information to motivational systems that can produce the indirect reciprocity behaviors that will serve as rewards to donors. However, the indirect reciprocity we’ve identified in our mallards suggests that those computational requirements may be fulfilled in vertebrates more commonly than theorists originally thought.

Neither of us could figure out for sure whether the focal ducks were transmitting information about our generosity/non-generosity to the indirect reciprocators through verbal (or non-verbal) communication, but we think it is unlikely. Instead, we suspect that the indirect reciprocators were directly observing our behavior and then using that sensory information to regulate their indirect reciprocity behavior.

In support of this interpretation, we note that on several cracker days, it was not only other ducks that engaged us as indirect reciprocators, but individuals from two different species of turtles (which we believe to be Rachemys scripta and Apalone ferox) as well. The turtles’ indirect reciprocity behaviors, of course, were different from those of the ducks, due to differences in life history and evolutionary constraints: The turtles didn’t reward our generosity through waddle-based or quack-based rewarding, but rather, by (a) rooting around in the mud where the focal duck had received the cracker earlier, and (b) trying to grab the focal duck by the leg and drag it to a gruesome, watery death. The fact that turtles engaged in their own forms of indirect reciprocity suggests that they, at least, were obtaining information about our generosity via direct sensory experience, rather than through duck-turtle communication or written or electronic records: It is widely accepted, after all, that turtles don’t understand Mallardese or use eBay.

The involvement of turtles as indirect reciprocators also suggests that indirect reciprocity might be even more prevalent–and more complex–than even we originally suspected. Not only does indirect reciprocity evolve to regulate interactions within species (viz., Homo sapiens), and between species (viz., between Homo sapiens and Anas platyrhynchos L., as we have documented here), but also among species (Homo sapiens as donors, Anas platyrhynchos L. as recipients, and Rachemys scripta and Apalone ferox as indirect reciprocators).

Finally, we should point out that although our results are consistent with the indirect reciprocity interpretation that we have proffered here, other interpretations are possible as well. We look forward to new work that can arbitrate between these two accounts (and perhaps others). We also see excellent opportunities for simulation studies that can shed light on the evolution of indirect reciprocity involving interactions between two or even three different species, which my co-Investigator thinks she might pursue after she has mastered long division.

h/t Eric P.

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Why Do Honor Killings Defy the First Law of Homicide? And Will Smaller Families Lead to Fewer Of Them?

Few categories of human rights violations more deeply scandalize the liberal (with a little-L) moral sensibility than honor killings do. Reliable numbers are hard to come by, but by most credible accounts it seems likely that several thousand Muslim women each year (and more than a few men) are stoned, burned, hanged, strangled, beheaded, stabbed, or shot to death for the sins of getting raped, falling in love, or dressing immodestly. But to anyone who thinks about human behavior from an evolutionary point of view, honor killings are not just morally outrageous: They’re also really puzzling.

As Martin Daly and Margo Wilson documented in their marvelous book Homicide, killers are very rarely the genetic relatives of their victims. Instead, they’re most often strangers, or rivals, or cuckolded lovers (who, of course, are not each others’ kin even if married—at least, not in the sense that matters to natural selection). Indeed, the typically low level of kinship between the victims of homicides and the people who kill them is so predictable that we could get away with calling it “The First Law of Homicide.” When two genetic relatives are involved in a homicide, it’s usually either as co-victims or co-perpetrators, not as victim and perpetrator.

In a sense, a general reluctance to harm or kill one’s genetic relatives is not exactly breaking news. We’ve understood since William Hamilton’s 1963 and 1964 papers that natural selection creates organisms that appear designed to maximize their inclusive fitness (which incorporates the reproductive success of the individual in whom the gene is physically located, as well as the reproductive success of other individuals who are carrying copies of that gene around) rather than their simple direct fitness. Genes “want” to maximize the total number of copies of themselves that are floating around in the world, even if some of those copies are located in other individuals’ gonads. The principle of kin selection virtually guarantees that we’re walking around with instincts that restrain us from harming our relatives, even when they’ve irritated us. To be clear, I’m not saying people never kill their kin (mental illness is a real wild card here), but the fitness disincentives of doing so were so high as our psychology was evolving that the perceived incentives to do so now have to be very high indeed.

Which is what makes honor killings so puzzling. In a recent article, Andrzej Kulczycki and Sarah Windle summarized data on the circumstances behind more than 300 honor killings across Northern Africa and the Middle East. What jumps off the page when you look at their data is how flagrantly honor killings flout the First Law of Homicide: About three-quarters of honor killings are carried out by family members of the victim. To be specific, the victims’ brothers carry out 29% of them, fathers and (to a much lesser extent, mothers), carry out about 25%, and “other male relatives” carry out an additional 19% of them. (Of the remaining 25%, virtually all are carried out by the victims’ husbands/ex-husbands.)

I’m really interested in that 75% that violate the First Law of Homicide. For the perpetrators of honor killings to over-ride their intuitive aversions to killing their own daughters or sisters, the perceived costs of “dishonor” must be very high indeed. We can’t precisely measure the exact fitness value of honor for someone who lives in a so-called culture of honor, of course, but the link between fitness and honor is undeniable. If you live in an honor culture, your honor determines your (and your children’s) job prospects, marriage prospects, ability to recruit help from neighbors, ability to secure a loan, and protection against those who would otherwise do you harm. Honor is an insurance policy, a social security check, and a glowing letter of recommendation rolled into one bundle. The fitness costs of tarnished honor in an honor culture can be steep.

One of the things I came to appreciate about honor while doing research for one of my books is that honor is a sacred commodity. It doesn’t follow the laws we expect actual physical stuff to obey, or the normal laws of economics, or even the normal rules that govern our everyday psychology. It follows the laws of Sacred Things. If you feel sad one day, you can be pretty sure that the feeling won’t last forever. Dishonor doesn’t work like that. Dishonor doesn’t wash off or fade away with time. Dishonor has to be purged or atoned for. More importantly for my argument here, dishonor does not dilute. The dishonor that a “dishonorable” behavior creates for a family is not like a fixed quantity of scarlet paint that can be used to make only a finite number of scarlet letters. When a young woman “dishonors” her family, there’s enough dishonor to thoroughly cover every one of her brothers and sisters, no matter how many brothers and sisters she has.

There’s an interesting prediction waiting in the wings. If I’m right that dishonor does not dilute, then the perceived fitness-associated costs of a single act of dishonor will be larger for a father and mother with many children than for a father and mother with only with only a few children. This has implications for reducing honor killings. Let me illustrate with a thought experiment.

The Costs of Dishonor to a Father Are Higher in Large Families

Say I am a father with nine children and one of my daughters has done something (or, more likely, has had something done to her) that has brought dishonor upon herself and each of her eight siblings. (Believe me, I am more appalled by having to write sentences like these than you are by having to read them, but I can’t come up with a better way to think through these issues than to try to step into the shoes of someone who is actually factoring honor-related concerns into their social decision-making). As the father of these nine children, the dishonored daughter has reduced my fitness by 9d because each of my children will suffer an honor-related fitness cost of d. (It might be better to quantify the hit to my fitness as 9 * .5 = 4.5 because my genetic relatedness to my children with respect to a rare allele that I possess is 0.5 rather than 1.0, but that won’t change anything in what’s to come. Can we please agree to work with 9 so as to make the math prettier?) So, if I am a father of nine children, and I can restore my family’s honor by murdering my dishonored daughter, I can recover 8d units of fitness (by restoring the damaged honor of my other eight children), and it costs me (I know, the thought sickens me as well) the fitness decrement I suffer through murdering one of my offspring.

If, on the other hand, I have only two children, then the perceived fitness cost of my daughter’s dishonor is 2d (a cost of d is imputed to both of my children), and I’d only be able to recover 1d unit of fitness (for my remaining, unmurdered child) by murdering the dishonored daughter. So, for a father with only two children, the calculus is not so clear: Am I better off in the long run to have two children whose honor is tarnished, or only one child whose honor is restored? For any plausible value of d, it’s hard to imagine that the decision-making scales would tilt in favor of killing the dishonored daughter if doing so would leave you with only one child. I’m betting that the father of two will stay his hand under circumstances in which the father of nine might not.

If I’m right about this, then a demographic shift toward smaller families in developing societies could eventually help to solve the problem of honor killings. I couldn’t find any direct evidence to support this prediction, but Manuel Eisner and Lana Ghuneim recently published a study in which they surveyed 856 Jordanian adolescents from 14 different schools to examine the predictors of their attitudes toward honor killings. They found that even when they controlled for the students’ sex (male vs. female), their religion (Muslim vs. non-Muslim), whether their mothers worked outside of the home (a good proxy for modernization), and the parents’ educational levels (also a good proxy for modern thinking), children with four or more siblings had more favorable attitudes toward honor killings than did children with three or fewer siblings. Not an exact test of my prediction, but to the extent that kids adopt their parents’ views, it seems to me that these results are at least tantalizingly consistent.

Do the human rights groups that want to reduce honor killings and other kinds of honor-related violence around the world ever talk about family size as a truly exogenous (and, in principle, modifiable) cause of honor killings? People are pinning their hopes for solving so many other problems around the world on reductions in family size, so perhaps I’m not being too pie-in-the-sky to add “reductions in honor-related violence” to that list of “Ways In Which We’d Be Better Off If People Had Fewer Kids.” As families shrink, I’m guessing that spared lives become subjectively more valuable than restored family honor.