London: Scientists believe there may be a valid reason why some women make excuses at times for not wanting to make love to their husbands.
They have discovered that the brains of females with a low libido behave completely differently from those with healthy sex drives.
Those low on libido don't have the same rush of blood to certain brain cells during passionate moments which could explain why they don't enjoy it, reports the Daily Mail.
The US scientists compared the brains of a group of women diagnosed with a form of low libido known as hypoactive sexual desire disorder with those women who have normal sex drive.
The team, led by Michael Diamond of Wayne State University in Detroit, US, asked them to watch erotic films and then a blank screen while they examined their brains using MRI scans.
Their study, which is being presented at the American Society for Reproductive Medicine's annual conference in Denver, Colorado, found that during the sex scenes, the women with a normal sex drive had an increased flow of blood to certain parts of their brain.
But those with low libido did not have any increase in blood flow, according to a Wayne State statement.
Some doctors claim that low libido is a complaint invented by drugs firms to cash in on female versions of Viagra.