The science behind the numbers that govern our lives

Think about your average day as a series of choices. You'll get up, you'll choose what to eat, whether to go for a run, whether or not to indulge in a glass of wine or a second helping of dessert. You're constantly making decisions based on what you want versus what you think is good for you. And how do you know what's good for you? Because somebody told you so. On average, we are given 123 different pieces of advice every week, from sensible government guidelines to scare stories about what gives us cancer. It all proves the point that you can assign a number to just about anything. Here's the latest thinking about the science behind the numbers that govern our lives.

"Have your babies before this clock strikes 12," Professor Mary Herbert, a specialist in reproductive biology, told an audience at the British Science Festival this year. "I would be getting worried about my daughter if she hadn't had a child by 35."

Herbert's Cinderella analogy may raise a few eyebrows, but she's certainly not the only expert keen to tell women who want to be mothers to get on with it. "I read one paper that referred to eggs as 'best used by 35'," says economist Professor Emily Oster.

"Thanks; it's really helpful to know my sell-by date."

Oster became interested in the "fertility cliff" when researching her book Expecting Better, which tackles the data behind the most common - and controversial - pregnancy advice.