Scientists have identified the secret of long-lasting love and claim they can tell which marriages will make it. To test this, four couples - two on the brink of divorce and two newlyweds - go through a series of experiments to see how being out of love affects how they physically see their partner and how their physiological reactions to conflict threaten love. With the new understanding of each other's biochemical and psychological behaviour, the couples embark on a programme designed to rescue their relationships.

Can science predict sexual attraction? In a ground-breaking experiment, a team of scientists orchestrate a unique speed date with a hundred singletons to test their theories - with unexpected results. The scientists analyse our speed-daters bodies, faces, and psychological and social profiles to find out what the key predictors of sexual chemistry really are. Looking for love is 32-year-old Beth Eagland. She's on a mission to find a husband but after four years of singles parties and surfing the net she has struck out. Peter Kerr is a 37-year-old politics lecturer who can never get beyond being just friends. Our scientists believe they can find them the perfect mate where orthodox match-making methods have failed.

Men and women are just the same. Just as caring, just as promiscuous, just as good at a three point turn. Aren't they? The results of a new BBC sex survey of 500,000 people from around the globe - the largest ever carried out - provide very revealing answers. And five men and five women are put through a unique battery of experiments to uncover the real differences between the sexes. Engineer Lloyd finds out what's missing in him. Bickering couple Liz and Craig squabble over who's the most empathic - until a stunning brain scan settles the argument. And in a television first we reveal the true effects of sex hormones on the brain by following 29-year-old Max as, under the influence of monthly testosterone injections, she turns from a woman into a man.