Where in the world are the fewest women on major corporate boards?

Japan, according to a study, out today, by Corporate Women Directors International. The Washington-based group, known as CWDI, counted women directors at the 200 largest companies on Fortune‘s Global 500 list.

As of year-end, Japan has the most companies with no women directors: 19. The all-male boards include Toyota , Honda , Nissan , Panasonic , and Toshiba .

What gives? Besides cultural stereotypes–age-old assumptions about the role of women in society–there’s the fact that in Japan, most board seats go to senior management. Since few women are senior managers in Japan, they have little access to the boardroom.

Though this may be changing, say the researchers at CWDI. Japan has moved to adopt certain Western corporate governance practices, including smaller boards and independent directors. Sony was a pioneer in recruiting outside directors. So it’s not surprising that Sony is a Japanese outlier, with two women on its board.

Lack of diversity likely handicaps Japanese companies, as studies suggest that the most effective boards are balanced, gender-wise.

Boards in the U.S. have made progress. One fifth of director seats at the largest U.S. companies, according to CWDI, are occupied by women. Though in terms of percentage of large-company director seats held by women, Norway scores best. That’s because the government has mandated 40% female representation at publicly listed companies.

Measuring board diversity this way–by percentage of seats held by women–Japan is not worst in the world, actually. What country is? South Korea. It has five companies in the top 200 of Fortune‘s Global 500. And the tally of women on those corporate boards: Zero.