The dramatic change in attitudes towards family life means couples are far more likely to cohabit, divorce or have a series of partners - and the study concludes that they are much happier as a result.
The findings come in a report for the National Children's Bureau and are based on a survey carried out two years ago of 17,000 people born in the same week in 1958.
It reveals that one-fifth of 33- year-olds had lived with two or more partners. Those happiest with their relationship were on second marriages: single women living with boyfriends were the least contented.
The study also suggests the continuing decline of the nuclear family. Seven out of ten people felt it was acceptable to have children without being married. Nineteen per cent of women and six per cent of men had a partner who was not the parent of at least one of their children. One-fifth of lone parents were single and seven-tenths divorced or separated.
Almost half of the mothers with children under five had no paid work, compared with 2 per cent of fathers in the same position. Half of lone fathers worked full-time but only a quarter of lone mothers. One-third of women worked part- time, against 1 per cent of men.
There were wide differences in educational achievement. Thirty- six per cent of those living in London had a degree, against 18 per cent in the North of England.
But another 4 per cent had trouble reading, 10 per cent had trouble writing or spelling and 3 per cent had numeracy problems.
Despite the 1975 Equal Pay Act, women earned less than men: the gross hourly earnings of women working full-time were 85 per cent of the rate earned by their male counterparts. The report found women also had fewer employment benefits.
Twenty per cent of women had shares in their firm, compared with 28 per cent of men. Fifteen per cent of women received private medical insurance, compared with 22 per cent of men. Only 8 per cent of women had a company car, against 24 per cent of men.
More than four-fifths of Britain's 33-year-olds were satisfied with their homes; 79 per cent owned them. But the picture was bleaker for separated, divorced or widowed women. Only 52 per cent were owner-occupiers, whereas 66 per cent of widowers owned their own home; 57 per cent of women had trouble paying their rent or mortgage, compared with 47 per cent of men.
Health was the major difficulty for thirtysomethings, according to the survey.
Two-fifths of men were overweight and 11 per cent obese, compared with 23 cent and 2 per cent when they were aged 23. Thirty- one per cent of women were overweight and 16 per cent obese, compared with 18 per cent and 4 per cent when 23.
Back problems were also widespread. Fifty-one per cent of men and 43 per cent of women claimed to suffer low back pain and. Almost a quarter of men and 15 per cent of women said it forced them to take time off work.
Life at 33: the fifth follow-up of the National Child Development Study; National Children's Bureau; pounds 13.50 (members pounds 8.50)