"Our data suggest that men's interest in children predicts their long-term mate attractiveness even after we account for how physically attractive the women rated the men," said James Roney of the University of California, Santa Barbara.

Researchers tested the testosterone levels of 39 male undergraduate students from saliva samples. Moreover, they showed them pairs of photos of an adult and of a baby and asked them to choose the one that most raises their interest. High levels of testosterone proved to be correlated to a lack of interest in babies.

But would women be able to guess that from men's faces? Researchers photographed the men and then showed the photos to 29 female students who were asked to rate the men on whether they thought they liked children, their masculinity, physical attractiveness and potential as short and long-term partners.

Women rated as masculine the men with the high testosterone levels and chose them for short-term relations. However the men they chose for long-term relationships proved to be precisely those that manifested more interest in babies.

Scientists argued that women can pick up cues about how men feel about children from their faces and use the subtle signs to rate them as potential partners. "The study provides the first direct evidence that women's attractiveness judgments specifically track both men's affinity for children and men's hormone concentrations," said Dario Maestripieri of the University of Chicago, and a co-author of the study.

Women are surprisingly accurate in being able to determine interest in children and testosterone levels, said James Roney, the lead author of the paper. "The research suggests that men's interest in children may be a relatively underappreciated influence on men's long-term mate attractiveness," he added.