SAN FRANCISCO, (Reuters) -- Russian cosmonauts who served aboard the Mir space station were generally happier and more satisfied than their American counterparts, according to the first-ever mental health study of crews and controllers in piloted space missions.

The study, conducted by University of California at San Francisco researchers, concluded that lopsided crew composition of two Russians and one American on each mission left U.S. astronauts feeling both frustrated and lonely.

"In multicultural crews, especially small crews, one has to pay a lot of attention to the culture and language background of the people involved,'' Nick Kanas, a UCSF professor of psychiatry, said Thursday. ``A single person who is different from the other two can feel isolated.''

It found unequivocally that the American participants were less satisfied with their group interaction and work environment than were the Russians, reporting less support and direction from superiors, more work pressure, less personal opportunity and less physical comfort.

Kanas said a major reason for the difference was likely the fact that on each mission, a solitary U.S. astronaut was teamed with two Russian crew mates.

"This creates a potential imbalance,'' Kanas said. "The commander was always a Russian; the language used was always Russian; and the operational control of the Mir space station was in Russian hands.''

Kanas, who is also associate chief of mental health services at the San Francisco Veterans Affairs Medical Center, said that the results of the study should prove useful to NASA as it plans future, lengthy missions -- from the International Space Station to potential missions to Mars.

"Any problems that exist are highlighted when you are confined and further highlighted when you are confined for months,'' Kanas said in an interview.

"One of the things we're recommending in the future is that crews be alerted to this before they fly ... that there are mechanisms for crew members to look at how they are relating in space.''

Three-person crews are likely to remain standard for some time, chiefly because the planned space station's Soyuz escape pod can hold only three people. Kanas' report stressed that care should be taken on future missions to avoid setting up an automatic "odd-man out'' situation, which could exacerbate tension among the crew.

"Three-person crews are tough because the number three, even when everybody is demographically similar, is a very unstable number,'' Kanas said. "And it always gets more difficult when you go into it with an automatic minority, either by language or culture or sex.''

Until crews can be expanded, Kanas suggested that officials set up a system whereby the leadership role can be rotated among the three crew members -- preventing the formation of on-board power blocs.

Kanas said the study also indicated that NASA should strengthen psychological training for astronauts, noting that while most U.S. space missions thus far have been relatively brief, future trips could send astronauts into space for months or even years with no outside support.

"On a trip to Mars, for example, there is going to be a time delay in communication, and any sort of medical problem that takes place is going to have to be taken care of by the crew itself,'' Kanas said.

"It is important that at least one of the members, and maybe more, have some therapeutic and psychological counseling training.''