It appears the turning point in a relationship is the first anniversary because it is then that a man’s testosterone level – the key to his sex drive – drops dramatically (stock image above)

For years, the test of a relationship was that it had survived the seven-year itch.

Now it appears the turning point is the first anniversary because it is then that a man’s testosterone level – the key to his sex drive – drops dramatically.

Researchers said that during the first year a man is poised to fight off competition from rivals while keeping an eye out for a more suitable mate.

But they found that after 12 months with a steady partner comes a ‘significant’ fall in testosterone.

The scientists interviewed 75 men aged between 18 and 39 about the state of their relationship and its duration.

They also tested levels of testosterone and found they began to fall off sharply after the 12-month mark – the end of a ‘new’ relationship. For those in a serious long-term partnership, hormone levels dropped by almost a third.

Lead author Dr Daniel Farrelly, of Worcester University, said the biological change may explain why the date has become significant socially as a marker of a couple’s commitment to each other.

‘Although such landmarks are not part of our evolutionary history, it would perhaps be adaptive for a man’s internal perception and evaluation of their current relationship to be affected by external cues such as this either consciously or not,’ he said.

The study in the journal Evolutionary Psychology added: ‘Results showed that levels of testosterone of single men and men in new relationships did not differ, but both had significantly greater levels of testosterone than men in long-term relationships.’

Dr Farrelly told the journal that lower testosterone levels could help relationships by making the man more suited to caring for children.

‘Men in new relationships are still in a physiological state that aids competition with other men for access to potential mates,’ he said.

'This may be due to the need to maintain and develop their new relationship or still being motivated to seek further mating opportunities… a status that may change should the permanence of the relationship become more established in the future.’

Lower testosterone levels could help relationships by making the man more suited to caring for children, researchers said

The psychologist from the university’s Institute of Health and Society said there were evolutionary reasons for the fall in male hormone being linked to commitment, adding: ‘Testosterone plays an important role in men when they are seeking mating opportunities with women.

‘However, a decrease in levels… when they are no longer seeking new relationships has clear adaptive benefits.