A happy, healthy relationship and sex life are all very well, but when it comes to measuring a woman's sexual desire, all scientists really need to know is how long a couple has been together, Daily Mail writes.

Women's sex drives gradually ebb over time, say the authors of a new study, while a man's stays at around the same level. In fact, on a desirability scale, women's yearnings decreased steadily with every passing month of a relationship, making it possible to gauge a woman's sex drive just by looking at a union's duration.

The study, published last month by the Journal of Sex & Marital Therapy, goes some way to addressing a subject area that is relatively unchartered and yet - as many a couple may attest to - is believed to form a key part of relationships.

170 men's and women's desire levels were monitored and rated on the Female Sexual Function Index. Participants, all in heterosexual relationships ranging from one month to nine years in length, were all undergraduates at the University of Guelph in Ontario, Canada, where the author's lead researcher, Sarah Murray, is based.

Ranging from 1.2 to 6.0, the scale quantifies sex drive so accurately that Ms Murray and her research partner Robin Milhausen found 'specifically, for each additional month women in this study were in a relationship with their partner, their sexual desire decreased by 0.02 on the Female Sexual Function Index'.

The findings were indicative enough to lead the authors to believe that relationship duration is a better predictor of sexual desire than satisfaction in bed or overall relationship health, reports Live Science.