Which Lens for Photographing People: Prime or Zoom?

Not so long ago, my answer to the title question would have come out almost automatically: a prime lens, of course! Just a few decades ago, zoom lenses suffered from a whole range of optical compromises that made their use for portraiture and documentary photography somewhat questionable. But today? If your budget allows, you can buy a standard zoom with an f/2 aperture whose optical quality rivals — or even surpasses — many prime lenses, and instead of carrying four lenses, you can get away with just one. And yet, the popularity of prime lenses doesn’t seem to be declining. Quite the opposite. So which one is better for photographing people?