Counting is so simple, so basic, so important. We counted the numbers of boys and girls on sugary cereal boxes, on the covers of board games, in the action section of toy aisles, in Newbury Award winning books, and we reported studies that counted the number of boys and girls in G-rated films, and other forms of media. This was our way of showing where the girls aren't, sure, but more importantly we did this because numbers give a clear and present message to girls (and boys) about who should be doing, wearing, listening to, reading, and playing with what. The results can have long-term impact. Consider a recent article in the journal Psychological Science (Vol. 18, Issue 10) called "Signaling Threat: How Situational Cues Affect Women in Math, Science, and Engineering Settings," by Mary C. Murphy, Claude M. Steele, & James J. Gross. Turns out the kind of low numbers we reported seeing in movies, TV shows, books, and so forth give "situational cues". The researchers found that simply watching a conference video in which women were outnumbered by men made the women-all math and science majors--feel like they didn't belong and feel like not participating. It also made them vigilant of possible threats to their identity. The situation they observed gave the young women that intangible "in the air" feeling that they were unwelcome and might be ostracized if they participate. Young men didn't experience the same threat. They were protected by the reality that they are almost always in the majority and expected to do well in these arenas, and so being in the minority this one time didn't phase them.
If girls see only one girl in a cartoon about geniuses or just one woman in the race for presidency - this gives them a very real and tangible message: you aren't welcome here. It also discouraged them from wanting to do the things they see primarily boys do and to be anxious, isolated, and feel out of place when they break boundaries. This is the reason to care about how media depicts girls and boys. We can no longer accept the lame excuse -- Girls will watch boys, but boys will not watch girls - used to justify the 75% male character rate in G-rated films. Yeah, maybe they will watch. But at what cost to them?