'Of course, the more education you have, the more likely it is that you'll find a good job, so an increase in education often leads to an increase in income.'

More education might mean that parents are more likely to read to their children, she suggested.

Or it could be that parents who are in school need to be more organised in order to get everything done, so they tend to create a more structured home environment, with dinner and bedtime occurring at regular times, for example.

Creating a more structured environment for children -- as opposed to giving them lots of free time -- has been getting something of a bad reputation lately.

But she believes that for the vast majority of American children, the value of free time has been exaggerated. 'There's this idealistic, nostalgic idea that free time gives children a chance to go out and play, and just experience nature,' she said.

'But in reality, in today's world where both parents are likely to be employed outside the home, what free time means for most kids is sitting in front of the TV, playing video games and generally being bored with no stimulation.'