(FORTUNE Magazine) – Here's why: -- Men's annual retirement income, including Social Security, pensions, and personal savings, averages $10,450 -- not much. But women get only $6,020. Since women live some seven years longer than men, they're more likely to outlive insufficient assets. -- Women typically earn less throughout their careers and thus have less to save. A 1992 Phoenix Home Life Mutual Insurance study of 1,500 people with household incomes over $30,000 found that 43% of the women and 68% of the men interviewed earned $40,000 or more. The women saved about $2,100 a year, the men $3,000. -- More women than men work in smaller businesses that do not offer pension plans. Only 18% of women over age 40 expect to or currently receive employer- provided retirement benefits, reports the Labor Department. In the Phoenix Home Life survey, 72% of men were covered by pension plans, compared with 59% of women. -- Women often leave and rejoin the work force to have children, move with spouses, care for elderly relatives, or cope with family problems. As a result, they may not qualify for company plans or fail to accumulate much in them. -- In the event of divorce or death, women may lose part or all of a spouse's retirement income. Yet the notion of living on a spouse's savings remains rooted. Says Fidelity Investments' Roger Servison: ''Women who aren't working have no Social Security of their own. They cannot contribute to an IRA account.'' Women can overcome these challenges, but it won't be easy. Princeton University economist B. Douglas Bernheim suggests that single women ages 25 to 44 earning $50,000 annually with a company pension set aside more than 10% of after-tax income -- twice that recommended for men. Still, women may have one advantage. Says financial planner Diahann Lassus of Lassus Wherley & Associates in New Providence, New Jersey: ''Men tend to take higher risks and want short-term results. Women approach investing for the long term and are more patient. And that's what counts for retirement.''