A map from UNESCO provides a very rough picture of "the gender gap in science" around the globe, showing large swaths of relative equality in parts of South America and Central Asia, and great inequality in countries including India, France, Germany, and Japan. In those places--some of the world's science powerhouses--women make up less than a third of researchers.
But, like I said, this is a rough picture. Its data comes from headcounts that include any professional "engaged in the conception or creation of new knowledge, products, processes, methods and systems, as well as in the management of these projects." What are the distributions of women and men at the entry-level jobs? What is it like at the tops of the most powerful hierarchies?
Additionally, the map does not show the U.S., which the report chalks up to "a lack of data." I tried to track down that data myself, but unfortunately, with the National Science Foundation website down due to the shutdown, I ran into the same problem (an archived version of the site does not include any of the Excel tables needed). (The map also unfortunately does not cover Australia, China, Canada, and a few other key countries.)
That said, Eileen Pollack's wonderful recent New York Times article can fill in a bit where the official statistics are wanting. She writes, "Only one-fifth of physics Ph.D.’s in this country are awarded to women, and only about half of those women are American; of all the physics professors in the United States, only 14 percent are women." Pollack tries to answer The Big Question: What, in 2013, "could still be keeping women out of the STEM fields (“STEM” being the current shorthand for “science, technology, engineering and mathematics”), which offer so much in the way of job prospects, prestige, intellectual stimulation and income?"