Signalling Theory - The Sports Handicapping Metaphor

The Sports Handicapping Metaphor

In 1975, Amotz Zahavi proposed a verbal model for how signal costs could constrain cheating and stabilize an "honest" correlation between observed signals and unobservable qualities, based on an analogy to sports handicapping systems. He called this idea the handicap principle. The purpose of a sports handicapping system is to reduce disparities in performance, making the contest more competitive. In horse racing, intrinsically faster horses are given heavier weights to carry under their saddles. In amateur golf, better golfers have fewer strokes subtracted from their raw scores. This creates correlations between the handicap and unhandicapped performance, and if the handicaps work as they are supposed to, between the handicap and handicapped performance. If you knew nothing about two race horses or two amateur golfers except their handicaps, you could infer which horse or golfer has had the better performance in the recent past, and which competitor is most likely to win: the horse with the bigger weight handicap and the golfer with the smaller stroke handicap. By analogy, if peacock tails act as a handicapping system, and a peahen knew nothing about two peacocks but the sizes of their tails, she could "infer" that the peacock with the bigger tail has greater unobservable intrinsic quality, in the sense that it is better able to pay the costs of displaying the tail (here, "infer" is shorthand for the idea that females that prefer bigger tails are at a selective advantage). Display costs can include extrinsic social costs, in the form of testing and punishment by rivals, as well as intrinsic production costs.

The essential idea here is intuitive and probably qualifies as folk wisdom. It was articulated quite nicely by Kurt Vonnegut, in his 1961 short story Harrison Bergeron. In Vonnegut’s futuristic dystopia, the Handicapper General uses a variety of handicapping mechanisms to reduce inequalities in performance. A spectator at a ballet comments: "it was easy to see that she was the strongest and most graceful of all dancers, for her handicap bags were as big as those worn by two hundred pound men." Zahavi interpreted this analogy to mean that higher quality peacocks with bigger tails are signalling their ability to "waste" more of something important by trading it off for a bigger tail. This resonates with Veblen’s idea that conspicuous consumption and extravagant status symbols can signal wealth.

Zahavi’s conclusions rest on his verbal interpretation of a metaphor, and initially, the handicap principle was not well received by evolutionary biologists. However, in 1984, Nur and Hasson used life history theory to show how differences in signalling costs, in the form of survival-reproduction tradeoffs, could stabilize a signalling system roughly as Zahavi imagined. Later in the decade, several papers using genetic models also began to suggest that the idea just might work, at least some times. The logjam was broken in 1990 by Alan Grafen, who developed a very complicated marginal fitness maximization model of evolutionary signalling games and came to the conclusion that, given certain assumptions, a handicap-like signalling system could be evolutionarily stable, if higher quality signallers paid lower marginal survival costs for their signals.

A specific and widely applicable handicap mechanism was proposed in 1982: parasite-mediated sexual selection. It proposed that due to a never-ending co-evolutionary race between hosts and their parasites, sexually selected signals are indicators of health. This idea led to an explosion of research on the relationship between sexually selected signals, parasites and mate preferences during the 80s and early 90s.

Since the early 90s attention has shifted slightly towards that idea that carotenoids have dual but mutually incompatible roles in immune function and signalling. Given that animals cannot synthesize carotenoids de novo, they must be obtained from food. The hypothesis states that animals with carotenoid-depended sexual signals are actually demonstrating their ability to "waste" carotenoids on sexual signals at the expense of their immune system. This hypothesis has been the topic of an extensive body of work (e.g.,.) These red, orange and yellow carotenoid-dependent ornaments are hypothesized to be a general form of an immunocompetence handicap, and a specific mechanism by which the handicap principle might work. But how can carotenoid-dependent sexual ornaments be both a general form and a specific mechanism of the immunocompetence handicap hypothesis?