Is social security less fair to Susan than it is to Sam?

Social security is living in the past.

While women march to work in record numbers and traditional families become more scarce, the social security system still distributes benefits as if housewife-and-breadwinner households were the norm. Many women and two-salary couples thus feel the system treats them unfairly.

The report of the National Commission on Social Security Reform (NCSSR), due Jan. 1, will contain a section stressing the importance of modernizing social security to reflect the social change of the last 30 years, commission members say. But the system's formidable financial problems will make it difficult for Congress to consider the issue immediately.

''It's imperative that we recognize that social security places many women at a great disadvantage,'' says former US Rep. Martha Keyes, an NCSSR member.

Social security was founded in 1935, when the work of most married women revolved around cleaning, cooking, shopping, and picking up after the children. In 1940, both husband and wife worked in only 3 of every 20 families.

Benefits were thus structured to reflect that most families had one breadwinner and one dependent spouse. Single retirees received a basic benefit; married retirees were paid the same base pension, plus an extra half-share.

But the typical family has changed tremendously since the days of the New Deal, with the most obvious difference being the flood of women into the work force. Today, both husband and wife work in more than half of US households.

The basic distribution of social security benefits, however, hasn't been altered. It's still suited to the era of Harry Truman, saddle shoes, and ''Father Knows Best'' - making the system relatively unfair to many working couples.

Unless husband and wife make close to the same amount of money, found the 1979 Advisory Council on Social Security, a two-salary couple will draw less from social security than a one-salary couple with the same total earnings.

The wife is typically the spouse directly affected, as most employed wives work part-time or at low-paying jobs to supplement their husband's income.

Another social change - a divorce rate that has doubled since 1950 - has affected the retirement-income prospects of many women. Divorced wives are entitled to no social security benefits unless they have worked long enough to earn them, or unless the marriage lasted at least 10 years. But two-thirds of all divorces now occur before the 10-year cutoff.

The commission, at its November meeting, agreed to include in its report a section recognizing the importance of women's issues. It will recommend no sweeping reforms, however, as it has no time to study the area in depth, according to NCSSR chairman Alan Greenspan.

Previous social security study panels have urged more specific action.

The 1979 Advisory Council on Social Security, for instance, spent more time on women's issues than any other topic. A majority of the council recommended Congress consider ''earnings sharing,'' under which each partner in a marriage would be credited with half the couple's combined earnings, regardless of who actually earned what.

Earnings sharing, say its proponents, recognizes the fact that marriage is an economic partnership. It would treat one-earner and two-earner couples equally, and give many divorced women better social security coverage, they say.

But, depending on how such a program was designed, it could cost up to $3 billion a year more than the current system. Many divorced men would find their future benefits slashed.

Other, narrower, possible solutions to women's issues include changing the social security system to allow working mothers credit for up to 10 child-care years, and labeling social security benefit credits as ''property'' to be divided at divorce.

But Congress, occupied by social security's daunting financial difficulties, has lately had little inclination to address questions of equity within the retirement system.

Proponents say they don't expect earnings sharing to progress much this year. They claim it will be a more visible political issue in 1984 or '85, after social security's financial house is put in order. Earnings sharing realistically could be phased in during the 1990s, say proponents, when social security's projected surplus could help absorb any extra cost.

''It's going to take a while to get something like this passed,'' admits NCSSR member Fuller.