As in social relations, girls seek harmony in making music
Compared to classical music, one huge letdown of popular music is the light emphasis on harmony. A great melody hooks you and pulls you along, but you get more of a feel for the sublime when you're pulled in multiple directions and somehow not getting torn apart by cacophony. Still, there's one reliable source for harmony in pop music -- girl group vocals.
Because females are designed to work in small groups of close friends and kin, compared to males who are made for interacting in large groups outside of their immediate family, females don't work well in dominance hierarchies -- that's not how two best friends or two close sisters treat each other. Males sniff each other out, figure out who goes into what tier of the pecking order, accept their fate, and then get on with the business of hierarchical teamwork. This pattern shows up across cultures.
When a group of girls gets together and decides to make music, they bring this anti-dominance bias along with them. As a result, you see a lot less difference in status in girl bands than guy bands. In the latter, there may be a lead singer who hogs all the attention, while the other two or three are practically invisible. In the former, you're much more likely to see the members sharing duties in songwriting, vocals, lead vs. rhythm guitar, and so on. No one wants to be "that girl" who tries to monopolize the spotlight, alienate the other girls, and break up the band. In interviews, The Bangles went out of their way to emphasize that there's no lead singer or chief songwriter -- that they were all equals (or close to it anyway). In the ones I've seen, it was typically Susanna Hoffs who leapt to make the point since everyone assumed she was the lead, as she was by far the cutest one.
The easiest solution to this dilemma is to not play instruments and just sing -- that way they avoid the specialization that leads to hierarchy. For this reason, vocals-only girl groups are by far more common and successful than ones where they play instruments.
At the same time, having three voices in the song doesn't solve the problem entirely: they still have to get along aurally. Hence the greater tendency toward harmonizing in girl groups.
This idea also helps us make sense of when we see more or less of the egalitarian-harmony arrangement. If the girl band is made up of fairly masculine females, as in punk or grunge, we expect their masculine minds to be more accepting of hierarchy and have a rock band that looks a lot like a guy band, complete with a lead singer who monopolizes all attention -- the Riot Grrrl bands, for example. The more R&B-oriented girl groups of the early 1960s, which drew feminine girls, are at the other end. Even among all-male groups, the more feminine Beatles did not have as much internal hierarchy and focused more on harmony than did the more masculine Rolling Stones.
The idea makes sense of another pattern -- namely, why mixed-sex groups always have the female in the lead role, from Blondie to Mazzy Star to The Yeah Yeah Yeahs. The lazy cynical answer is that record producers and the band themselves do this for financial gain -- if there's a cute chick fronting the band, that will draw a larger audience and bigger bucks. Of course, why they use this tactic and not any other, probably more profitable, way to sell out is never explained. In reality, it is because females don't tolerate dominance hierarchies well, so they'll be more resentful than guys will of being that lower-status band member who nobody knows about. And she doesn't even have to feel resentful -- just being more nervous and likely to buckle when there's a leader giving you orders would be enough to make you want to bail.
So, because there's no other spot for someone with an anti-dominance bias than the lead role, that's where girls end up in mixed-sex groups. This pattern also proves how free of sexism the pop music industry is -- if patriarchal male rule prevailed, they'd assign her the least prestigious role instead of making her the lead virtually without exception. Somehow the hand-wringers seem to have missed that point.