The higher you are on Britain's social ladder, the more likely you are to have had more than one sexual partner in the past year. And in France young people are more likely to use condoms than their elders.
These are among the findings of new studies of sexual activity in France and Britain.
The researchers, who reported their findings in the current issue of the journal Nature, said their research was the first and most comprehensive large-scale examination of sexual behavior in the two countries. They said they hoped the work would help in efforts to prevent the spread of AIDS. Multiple Sexual Partners
In each country, at least twice as many men as women reported having more than one sexual partner in the last year. In Britain the figures were almost 14 percent for men and just under 7 percent for women; among the French, the figures were 13.3 percent for men and 5.6 percent for women.
In the British study, 6.1 percent of men reported having had a homosexual experience and 1.4 percent said they had had a homosexual partner in the last five years.
The French researchers, who also reported on homosexuality among women, said 4.1 percent of men and 2.6 percent of women reported having had sexual relations with a person of the same sex. Only 1.1 percent of men and three-tenths of 1 percent of women reported having relations with a person of the same sex in the past year. The proportion was almost five times higher in Paris than in rural communities.
"The AIDS epidemic has focused our need to collect information on sexual attitudes and life styles in order to predict and prevent the spread of H.I.V.," Kaye Wellings, a sociologist at St. Mary's Hospital Medical School in London, said of the human immunodeficiency virus, which causes AIDS. Ms. Wellings was one of four researchers who conducted the British study.
The survey was led by Dr. Anne M. Johnson, an epidemiologist at University College, London. Interviewers questioned more than 18,000 men and women in person about their sexual behavior. The people, who ranged in age from 16 to 59, were randomly chosen from a list of every household in England, Scotland and Wales.
Dr. Alfred Spira, director of Analysis of Sexual Behavior in France, a research consortium financed by the French Government, led the investigation there. More than 20,000 people from 18 to 69 were randomly selected from lists of households with telephones and interviewed by telephone.
The French researchers found that more than 56 percent of men and 43.7 percent of women said they used a condom during intercourse at least once in the past year. The most widespread use of condoms was among 18- and 19-year-olds, with 79.8 percent of men reporting at least one condom use, as against 48 percent of women.
Condom use was more prevalent in people with several partners. Among 18- to 44-year-olds, about 75 percent of homosexual men with multiple partners and more than 65 percent of women with multiple heterosexual partners used condoms at least once in the last year.
"The most surprising finding for me," Dr. Spira said in a telephone interview, "was the relatively high use of condoms among young people, especially during their first experiences in intercourse, though it decreases as people become more accustomed to sex. There seems to have been a real increase in condom use in France, but unfortunately it is not enough because about one-third of men and half of women at risk of contracting H.I.V. or sexually transmitted diseases never used a condom during the last 12 months."
The findings should help public health officials calculate more precise estimates of the number of people likely to become infected with H.I.V. and to better direct prevention efforts, the scientists said. More than 18,000 cases of H.I.V. infection have been reported in Britain and about 25,000 cases in France.
"Our results clearly indicate that for prevention purposes you must carefully take into account demographics," Dr. Spira said. "Sexual behavior is not the same among men and women, young and old."
In the French study, respondents were assured in a letter that all answers would remain anonymous and that information identifying them would be destroyed once the first answer was entered into a computer. The British relied on 500 experienced interviewers who met individually with the subjects. Findings in Britain
Among the British findings were these:
*The proportion of people reporting 10 or more partners so far in their lifetime is higher among people from 25 to 44 than among those over 45.
*A quarter of men under 25 reported five or more partners in the last five years.
*People in the professional and managerial classes were twice as likely as manual workers to report having two or more partners in the last year.
*More than one in five men and one in four women with five or more heterosexual partners in the last five years reported having had a H.I.V. test. Rates of testing declined in those 45 and older. Among men with homosexual partners, the proportion rose to more than 4 of 10.
"It's good news that people in the highest risk groups seem to be getting the message about H.I.V. and have gone on to seek advice and testing," Dr. Johnson said.
In the French survey, just over 3 percent of men reported intercourse with prostitutes over the last five years, with the practice much more frequent in Paris than in rural areas.
The proportion of French men having their first intercourse with a prostitute has sharply decreased in the last 20 years. The figure was 10 percent for men now 45 to 69, compared with only 2 percent for those 20 to 24.
Although the researchers said the level of condom use was higher than they had expected, they noted that a high proportion of people did not use condoms. The survey found that one-third of heterosexual men, half of the heterosexual women and one-quarter of men who were homosexual or bisexual did not use condoms.
"The level of protection currently used is insufficient to halt the spread of the H.I.V. epidemic," the French report said. The British survey also asked about condom use, but those results were not included in the initial report.
The full results of both surveys are to be published next year. The scientists said they could not make direct comparisons between the studies until then, though Dr. Johnson said she was struck by the similarities.
The British survey, which cost about $1.7 million and started in 1990, was financed by the Wellcome Trust, the country's biggest private supporter of biomedical research, after the Government of Margaret Thatcher, then the Prime Minister, refused to provide financial support. The French research, which is continuing, has cost at least $2.5 million so far. It is publicly financed.