The differences in what men and women look for in a long-term partner have narrowed to a degree over the course the last two decades, but overall mate preferences continue to reflect evolutionary principles, according to a study published in the journal Evolutionary Psychological Science.

According to theory in the often-controversial field of evolutionary psychology, men and women are attracted to different traits in the opposite sex because of differences in effective reproductive strategies. From the evolutionary perspective, men’s primary challenge is to identify a mate who is physically capable of bearing healthy children. Therefore, men tend to be attracted to indicators of fertility, including youth and aspects of physical beauty related to good health (such as an even skin tone and facial symmetry). From the same perspective, women’s main concern is identifying a mate with the ability and desire to provide for the material safety of her children.

As a result, women are thought to be attracted to men with signs of social and economic security (such as status and wealth) and physical traits related to the ability to be economically productive (such as strength). A number of major studies have found sex differences in mate preferences reflecting these predictions, but it has also been suggested that these differences have grown smaller over time as a result of growing gender equality.

Two psychologists from the University of Amsterdam, Jens Bech-Sorensen and Thomas Pollet, sought to study changes in sex differences in mate preference by replicating a classic study in evolutionary psychology. The original study, published in 1994, assessed differences in men’s and women’s preferences for marriage partners on 12 dimensions (such as age, attractiveness, and sexual experience) in a large nationally-representative US sample. The new study revisited the same questions twenty years later in a sample of 522 US residents recruited on the internet.

The extent of sex differences was significantly smaller in the new sample than in the old sample on four dimensions. Men were more likely than women to be willing to marry someone 5 years or more younger than themselves, and also more willing to marry someone earning less money than themselves, in both 1994 and 2014, but the size of both of these gaps was smaller in 2014. In 1994, men were more likely than women to say they would marry someone of a different race or to marry someone with less education than themselves, but by 2014 there were no differences between men and women on these dimensions.

Sex differences increased on only one dimension. Women were more likely than men to say they would marry someone who had previously been married, while there had been no difference between men and women on that dimension in 1994. The largest sex differences at both times were for age and employment preferences, with women continuing to prefer older and employed mates, and men continuing to prefer younger mates.

The study authors conclude that evolutionary factors continue to influence how people choose long-term partners, but that social change has probably influenced them as well. While the debate over applying evolutionary principles to human mate selection will continue, the evidence is growing that both biology and culture play a role determining what people find attractive.

“Our findings are in line with the argument that evolved mate preferences exist, while also highlighting that these preferences are likely malleable to socioeconomic temporal trends,” the researchers concluded.