There is a plot element sometimes used in high-school/coming-of-age movies (e.g., “Easy A”): the hero or heroine, wanting to appear more desirable, convinces a friend to pretend to have gone out or had sex with them. This gets them the attention they wanted but things inevitably get complicated and emotional angst, hilarity, and catharsis ensue (not necessarily in that order). As unrealistic as this plot ingredient may seem, the basic premise behind it is actually a well-known biological phenomenon: being seen to be desired really does make you more desirable.
There’s a neat evolutionary explanation for why this happens. Animals that want to have good-looking and successful offspring – which is all of us – can’t just settle for any mate. You need a mate that is themselves good-looking, as well as a good provider and an all-round nice guy (it is usually a guy; in most species it’s the females that choose who to mate with). Winnowing out the closet psychopaths takes a lot of time and effort, though, which is why dating is so complicated. However, there is a way to cheat: if you see a potential mate who is already with someone, you can assume that they’ve already been vetted by that someone. That person on their arm – which should really be you – has already done all the Googling, Facebook stalking, and cold-calling of their exes that passes for getting to know someone in the digital age. So, instead of doing all that work yourself, you can just copy their choice and go after the same (or a very similar) person.
This phenomenon is called “mate-choice copying” (or, more awkwardly, “non-independent mate choice”) and it happens all across the animal kingdom, from birds to mammals and, yes, also fish. It even occurs in humans, which is why we need a commandment to not covet our neighbour’s wife (as Ursula Franklin once pointed out to me, the Bible only bothers forbidding things people actually want to do; there is, for example, no explicit prohibition on cannibalism). We humans, who like to tell ourselves that we really care about compatibility and shared political views in our relationships, often copy mate choices and are actually more likely to copy the choice of a member of our own sex the more attractive they are .
Most of the work on mate-choice copying in fish has been done using guppies. Guppies are particularly well suited to these experiments because females that are making independent choices evaluate males on a lot of different features, including the size of their tails (which does matter) and how often they flash them around , the brightness of the red spots on those large tails, and how bold the males are . Recently, it has been shown that the personality of the choosing female also matters: more sociable females copy mate-choices more (; full disclosure: I was one of the authors on this paper).
The way these experiments are usually done is remarkably similar to the movie plot element described above (see picture at top). A female (usually) is allowed to demonstrate a preference for one of two males (movie: popular quarterback surrounded by cheerleaders; cut to shot of bespectacled nerdy kid in a plaid shirt, alone). Then, the female gets to observe the male she didn’t choose interacting with another female (the hottest cheerleader tells her friends in a loud voice how great the nerdy kid is) and observe the male she did choose alone. The other female is then removed and the subject fish gets to choose again. In a number of experiments like this, the subject female often reverses her choice (she now prefers the nerdy kid; e.g., ).
As I mentioned above, this is usually done with the test subjects being females, since they are most often the choosing sex, but mate-choice copying by males has been observed in both humans  and Sailfin mollies (which are closely related to guppies; ).
I mostly study social and collective behaviors in fish and one question I am frequently asked is what that could possibly have to do with human interactions. Well, they are very closely related, and mate-choice copying – which exhibits such clear similarities across species – is the perfect example of this. Living in groups presents unique challenges and opportunities that must be solved/seized by their members, whatever species they happen to profess. Fish or human, we’re all just chasing tail.