The traditional American family will be in the minority by 1990, researchers predict, and the number of people living alone will increase dramatically.

"By 1990, no one type of household will be 'typical'; the nuclear family consisting of mom, dad and the kids will no longer sway as it once did," according to a study by Harvard and MIT researchers.

In 1960, said Harvard Prof. George Masnick, 75 percent of all American families consisted of a married couple with children. By 1990, he predicted, half the population will be living alone, and of the married half, only half again will elect to have children.

Mary Jo Bane of the Harvard-MIT Joint Center for Urban Studies noted that, during the '80s, 20 million new households will be added to the nation's current 75 million, and of the new ones, 16.5 million will consist of unattached individuals. In 1990, she predicted, 60 million households will not have children under 15, more than the total number of households that existed in 1970.

Behind these trends, the report says, are an increasing preference for living alone, an increasing number of widowed or divorced people and a growing tendency among young people to delay marriage.