Researchers took a sample of women who could not perform a pull up and increased the strength of the biceps and lats (muscles involved in a pull up), as well as decreasing their body fat percentage. Surprisingly, after an increase in strength of 36%, only four of the 17 women could complete one pull up. The reason why? Researches cited "lower levels of testosterone" as the culprit...
"Men and women who can do them tend to have a combination of strength, low body fat and shorter stature. During training, because women have lower levels of testosterone, they typically develop less muscle than men, Vanderburgh explained. In addition, they can’t lose as much fat. Men can conceivably get to 4 percent body fat; women typically bottom out at more than 10 percent."
The title of this article "Why Women Can't Do Pull Ups" is a bit misleading. One of the problems I have with it, is the way the subjects trained; strengthening the biceps and lats. Sure, the biceps and lats are involved in the pull up, but the principle of specificity states that our body will adapt SPECIFICALLY to the stresses that are placed on it.
Example: If you want to get better at science (pull ups), you don't just study math (bicep curls and lat pull downs) even though that might help your overall knowledge (strength), you study (practice) science (pull ups)!
All in all, just because a research article says that "women can't do pull ups," don't use this (if you are a woman) as an excuse as to why you can't do a pull up! Train specifically to complete a pull up (assisted pull ups, negatives, chin hangs), and you will eventually be able to complete one!
The researchers in this study are much smarter than I am, but I wanted to share my two cents.
Link to the article: http://well.blogs.nytimes.com/2012/10/25/why-women-cant-do-pull-ups/