The one big mistake most workers make

Most Americans are failing to take advantage of a simple step that could lead to higher earnings.

Three out of five workers fail to negotiate their salary offers, according to new research from employment site Glassdoor. That means many are walking away from more money, potentially lowering their earnings for years to come. The problem is worse for women than for men, with 68 percent of women reporting that they accepted the offered salary without trying to negotiate for more. About 52 percent of men said they didn't negotiate.

The gender split may partially explain why women generally make less than men for doing the same job -- the so-called gender pay gap. Women in America are making just 76 cents for every $1 earned by men, Glassdoor found earlier this year. Even after adjusting for variables such as age and education, the employment site found that women still face a pay gap, earning more than 5 percent less than men.

While Glassdoor's most recent study didn't ask people why they decided against negotiating, the fact remains that workers should arm themselves with data to back themselves up during negotiations. That means understanding what professionals with similar credentials are making, for instance.

"It's important to understand that an offer is an offer. Come to the table with expectations that you will negotiate, but you want to do that with real numbers," said Glassdoor spokeswoman Jessica Jaffe. "If you don't ask, you won't get it."

While people regardless of gender should negotiate after receiving an offer, Glassdoor has specific tips for women, including doing your homework on pay rates within your industry, your city or region, and the company that's making the offer.

"Having data is helpful if you're going to make the case about why you should be offered more," Jaffe said.

Women may want to ask their current employers if they have a policy on equal pay for equal work, Glassdoor noted. Corporations are increasingly making efforts to ensure male and female colleagues with similar experience and educational backgrounds are paid at parity. Glassdoor noted that about 1,7000 employers have taken its pay equality pledge.

Women aren't the only workers who are more likely to skip negotiations, Glassdoor found. Older workers, or those between 45 to 54 years old, were less likely to negotiate than younger workers between 35 to 44, at about 66 percent to 55 percent, respectively.

"With mortgages and families and partners more likely, they're more likely to be conservative," Jaffe said of the 45- to 54-year-old age group.

Does negotiating pay off? About 10 percent reported success in their negotiations, Glassdoor said. While that's not great, if those workers hadn't negotiated, they wouldn't have received more money.