Mate-Choice Copying in Humans

One Short Announcement: Partly because I’ve been a little lazy of late, but also because my readership has changed quite a bit since starting my blog at Psychology Today, I have found myself a little negligent on the methods sections of my recent blog posts. I am uncomfortable with this, which is why I’m introducing a new format to my posts. Starting with this one. 

Recognizing that the larger share of my audience is more interested in the general story of a research paper than any of its scientific details, I want to try splitting my posts into two sections. One section will be a fun piece like the one below, the other will be (an even more fun) appendix that discusses one particular methodological aspect of the particular research paper for that post. For this post, I look at the basics of Repeated Measures ANOVA that Yorzinski and Platt  use on their mate-choice copying data. The Appendix can be found here.

Mate-Choice Copying in Humans:

Guppies are small fish with brains of only a few millimeters in width, and so one wouldn't expect them to make complex mating decisions: Take for example the typical female Guppy who has to choose between two males; one a little drab, the other brightly colored. For the typical female this situation presents a simple - practically hard-wired - call, and she will generally choose the brightly colored male. This is simply how things work in the Guppies' underwater world. (Note: Fish are much smarter than we usually give them credit for, so I'm cringing a little at my own use of the term "hard-wired" here...don't take it too literal).

Things do change sometimes in Guppy-world, however. For example, if another female enters the picture and, for some unspecified reason, this other female seems to favor the drab male.
When this happens, our typical Guppy female will often change her mate preference based on the simple observation that another female is actually mating with the previously undesired male. Seeing that another female wants this particular male seems to make him more attractive. A phenomenon called mate-choice copying, which takes place not only with Guppies:

A number of studies on human mate selection suggest that we might also engage in our fair share of mate-choice copying when faced with our version of having to choose between the brightly colored vs. the sadly drab potential mate. However, until recently only relatively little was known about the mechanisms that guide this type of mate-copying behavior. Surprising, given the potentially quite fascinating influence this type of behavior may possess for the evolution of sexually selected traits in populations.

An interesting study, trying to develop a better understanding of what cognitive processes may be driving mate-choice copying is one by Jessica Yorzinski and Michael Platt that was recently published in the academic journal PLoS ONE.
For their study Yorzinski and Platt invited thirty men and thirty women to rate pictures of potential mates. The pictures that each participant was given to rate were specifically photographed for this study, and they typically showed either a single man or woman, or a man and women together.

In the case that a man and women were shown together, the researchers played around with Photoshop a little, so that each male and each female model would appear on exactly three different "couples-pictures" in the study's photo-database. On one of these couples-pictures, the model was matched with a highly attractive person, on another the match was a moderately attractive person, and on the third matching was performed with a less attractive person.

Yorzinski and Platt now wanted to see whether attractiveness ratings, and even the invited rater's reported willingness to engage in a romantic relationship with the photographed models, would be influenced by whether pictures showed models on their own, or together with another person (since the study included all reported heterosexuals, couples pictures always included two opposite sex models).

Additionally, Platt and Yorzinski used a so-called eye tracker to collect data on where participating raters directed their gaze while viewing the pictures. In the case of female raters, they wanted to know whether female raters - when making a decision regarding a male model in a couple's picture-, would spend significant time viewing the female model that appeared in the picture, but more importantly, whether the presence of another female in the picture increased the attention given to the image of the male model. For men, the analog questions were of interest.
The graph below demonstrates one of Platt and Yorzinski's main findings: When participants of either sex rated the attractiveness of a potential mate in a "couples-photo", how attractive they perceived the potential mate to be increased with the attractiveness of his or her fellow-model in the photograph. The same person, when shown together with a very attractive person looked more desirable than when shown alone. However, when shown with a less attractive person the potential mate appeared to loose desirability.

The eye-tracking data also showed interesting results. For one, female spent more time looking at a man when he was shown next to a very attractive woman. Men did not show this type of bias to a statistically significant degree. However, Yorzinski and Platt also found that

"men and women differed in their gaze patterns while they evaluated the same-sex partners. The amount of time spent looking at partners influenced the women's mating decisions but the number of times looking back and forth between the partner and mate affected the men's mating decisions."

As Yorzinski and Platt write

"We found that proxy mating decisions made by people were strongly influenced by the attractiveness of partners depicted with potential mates. Specifically, men and women were more likely to express interest in a long-term relationship with a potential mate when that mate was paired with an attractive partner [...] We found that men and women differed slightly in their mate-choice copying behavior. Women showed an overall greater reliance on the decisions of same-sex partners than did men, although both were influenced by partner attractiveness. This pattern was especially prominent when the attractiveness of the same-sex partner was low: women were less interested in engaging in a long-term relationship with the mate while men's interest in the mate was not different from their initial evaluations. Because females are generally more selective in their choice of mates compared to men (due to differential parental investment) they may be more skeptical of mates paired with unattractive partners while males may have a high baseline interest in all potential mates."

Regarding their eye-tracking data and the observed gaze pattern differences, Yorzinski and Platt offer interesting possible explanations:

"[...] gaze differences could reflect differences between men and women in processing visual social information. Because men can process information about attractiveness faster than women, they may be able to gather information about same-sex partners with brief gaze shifts. Alternatively, shifting gaze could reflect men's vigilance, which may vary with the presence of a partner and his attractiveness, and thus index intrasexual competition."

...

On a closing note, I really hope that the guys from OKcupid read this and then take a look at what their admirable dataset says. Then again, I doubt that many people put pictures up onto online dating sites that show them with their attractive opposite-sex friends...

Note also that the original study includes a number of controls not mentioned here. It also goes into further detail regarding the interaction effects between attractiveness of the associate in the couples-pictures and reported willingness to enter into a relationship. The paper is available at PLoS One, and linked to below.

Main Reference: 

Yorzinski JL, & Platt ML (2010). Same-sex gaze attraction influences mate-choice copying in humans. PloS one, 5 (2) PMID: 20161739