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.