Sex matters to couples until it doesn’t, according to new research that finds happiness peaks when couples have sex once a week but does not increase with greater frequency.

“Although more frequent sex is associated with greater happiness, this link was no longer significant at a frequency of more than once a week,” said lead researcher Amy Muise.

Noting that research and popular culture have long suggested a linear correlation between sex and happiness – the more sex, more happiness – Muise and her colleagues at the University of Toronto Mississauga asked “Is it true that one can never have enough?”

For Americans whose survey answers they studied, “enough” amounted to sex about five times a month. Muise and her colleagues concluded that the relationship between sex and happiness was not linear but rather, appropriately, curvilinear. Their findings were published in the the journal Social Psychological and Personality Science.

“This is the first study to look at a curvilinear association between sex and well-being – so yes, the first to look at the ‘limits’ or ‘levelling off’ of this association,” Muise told the Guardian.

“Other work – on non-sexual topics – has shown that sometimes seemingly positive things do not uniformly increase well-being – for example, socializing with friends.”

In one study the researchers analyzed survey responses from 1989 to 2012 about sexual frequency and happiness, from more than 25,000 Americans in all. The findings were consistent regardless of gender, age or the length of the couple’s relationship.

The findings were specific to people in relationships, and researchers noted they found no association, linear or otherwise, between frequency of sex and well-being.

“There is not a lot of information about who the single people were having sex with,” Muise said, noting that some could have been in kinds of relationships. “Looking at when and for whom having more frequent sex when single is beneficial is another area ripe for future research.”

The researchers could not make any concrete claims about the reasons why sex more than once a week does not make couples happier than only once. Muise suggested that a follow-up study could look at whether couples have sex less than once a week become happier if they have more, or that satisfaction might be related to people’s notions of ideal sexual frequency.

“The big problem with this kind of work is that it’s correlational and not causal,” George Loewenstein, a professor at Carnegie Mellon University who was not involved in the research, said.

“People are basically having as much sex as they want, and for some reason it seems like couples who have found an equilibrium of about once a week seem to be happier. They didn’t control the independent variable, which is frequency of sex.”

Earlier this year Loewenstein led a study in which 32 of 64 married couples were asked to double their rate of intercourse and respond to short daily surveys about happiness. On average they managed to have 40% more sex, but not only were they not happier but found themselves feeling less energetic and having worse sex. Loewenstein said the lesson was simple: quality matters more than frequency.

“Sex can mean so many different things: there’s good sex, bad sex and we found that the quality of the sex affected happiness more than frequency. They didn’t have a measure of quality of sex.”

He noted that his studies run into limits of its own design. Researchers can find a curvilinear relationship with exercise or brushing your teeth, for instance. “And ours was worse in one sense,” he said, “it’s an artificial situation of someone telling you how much sex to have.”

Muise and her colleagues also compared the happiness brought by sex with that satisfaction of money, comparing reported well-being for each category. People who had sex less than once a month were less happy than those having sex once a week, the average, as were people who made $15,000-$25,000 a year versus $50,000-$75,000. The difference was greater regarding sex.

“People often think that more money and more sex equal more happiness, but this is only true up to a point,” Muise said.

The context of relationships likely affected survey responses, the researchers said, as did the benefits of sex that carry over one day into the next.

“The current set of studies help dispel the notion that sex has limitless benefits for well-being,” they wrote. “It’s important to maintain an intimate connection with your partner without putting too much pressure on engaging in sex as frequently as possible,” Muise added.