Top women executives do slightly better at U.S.-based healthcare and pharmaceutical companies than in other industries, but 35 percent of big drug companies do not have a single female director, according to a new survey.

The survey by Corporate Women Directors International found that 16 percent of directors at U.S.-based healthcare and pharmaceutical companies are women - slightly more than the national average for all companies of just under 15 percent.

But nearly 36 percent of Financial Times 500 companies did not have any female directors, including industry giant GlaxoSmithKline, the report found.

While we see promise among some companies - particularly those based in the United States - there is a shocking number of companies with absolutely no female representation at all, said Irene Natividad, co-chair of Corporate Women Directors International.

They are on boards. There are just not enough of them, Natividad said in a telephone interview.

It is so clear if you didn't have women there wouldn't be an industry. They are the arbiters of the health care, they are the workers. So why aren't there more? she added.

So for a company to grow, they really, really need to have people on the board and in management who have insights into how to reach that group better.

The survey found that Wellpoint, a health benefits company, had the highest percentage of women directors at 31.3 percent or 5 out of 16 directors.

Health insurers Aetna Inc., Britain's drug giant Astrazeneca and diversified healthcare company Johnson & Johnson all had boards of directors with just under a third female members.

None of the 22 Japanese healthcare and pharmaceutical companies included in the Financial Times 500 listings had a any women on their boards, the survey found.

I think there is a lack of vision on the part of current board members, a majority of whom are male, Natividad said.

It does require leadership from the top to make change happen.

Natividad said studies had shown that companies with more diverse board members - females and minorities - often had better share prices and overall better value.

What we are asking for is institutional change - it's cultural change in the business community and change is difficult for a lot of people, she said.

Last month the National Academies of Science looked at whether there were any good reasons for men to outnumber women in top scientific and mathematics faculty jobs and found none.

A committee of experts looked at all the possible excuses - biological differences in ability, hormonal influences, childrearing demands, and even differences in ambition - and found no good explanation for why women are being locked out.

Bias was the best possible explanation, the committee said.