"Bad enough to read the dismal science in the dog days of February," she said. "But in the festive season? Get a life."
Freakonomics, though, is an economics book with a difference. Like the work of the New Economics Forum in the UK, it recognises that, while conventional economics are good for some things, they are not so good at others.
Grand theory is eschewed. Economics, Levitt and Dubner suggest, is about the world as it actually is, morality and politics about how we might want it to be. And to my mind there is no doubt that adult learners' interests can be well served by the kind of divergent thinking Levitt and Dubner develop in a book full of memorable stories.
Their technique is to ask unusual questions, and to look for data that will help in answering them. So, while mayor Giuliani and his chief of police have been celebrated for their policy of zero tolerance of crime in New York, Freakonomics notes that crime rates were dropping in the USA before those innovative policies were introduced. Once the increase in police numbers was factored out, it concludes, New York's crime reduction was in line with national averages.
What, then, might account for the drop? The authors' analysis suggests that the major factor was the legalisation of abortion some 20 years earlier, reducing dramatically the number of unwanted poorer children born, and cutting the number of people most likely to be recruited to crime.
Other chapters ask why crack dealers live at home with their mothers (apart from a few, they do not earn enough to move out), and note how much higher the risks are of a child facing a fatal accident when visiting a friend with a swimming pool in the garden than a friend with a gun in the house.
They note that many of us have a fear of flying. More people die on the roads than in aeroplane crashes. But if you look at the hours spent travelling, road and air travel offer broadly comparable levels of risk, and both are safer than travelling on water.
Why, then, the fear of flying? Freakonomics suggests that we worry about risks where we have no control over what happens far more than those equal or greater risks where we have agency ourselves.
It is instructive to apply that principle to government thinking about lifelong learning policy. Everyone agrees that it makes economic and social sense to increase the skills base of the British workforce. As the interim report of the Leitch review of skills needs makes clear, increased investment in people with high, intermediate and low-level skills will be needed right through to 2020.
There is increasing acceptance, too, that while qualifications are the best proxy we currently have for the skills of the labour force, they are not an adequate one.
Employers highlight the need for teamworking skills, communication and problem-solving. Workers tell us that they like to learn incrementally and informally from their peers or managers, and that formal instruction has a place, but one much less central than our policy planners presume.
We also know that many of the jobs we will be doing in 15 years' time have yet to be invented, making it hard to train for them. Why, then, is the welcome investment in strengthening the skills of workers and potential workers below technician level focused exclusively on the achievement of "full fat" level 2 (GCSE equivalent) qualifications, often narrowly focused on technical skills fit for today's, rather than tomorrow's, economy?
Reading Levitt and Dubner, the suggestion occurs that we focus on the level 2 target because it can be measured, whether or not it is entirely fit for purpose.
For many in the workforce, this will result in no more than a confirmation of the skill levels workers already possess. Others will be excluded as a result of the inflexibility of the timing and structure of our qualifications, and the credit system to counter this still seems years off.
What is to be done? Sir Mike Tomlinson's report offered a major clue.
Re-thinking the vocational curriculum for adults and young people alike would make for a good start, offering vocational students the breadth of studies we take for granted in schools and in higher education.
Trusting learners would help, too. As Foster recognised, they are central to the system, and should be listened to. They often understand better than the educators what studies will make a difference to their lives.
But of course, that strategy brings risks. Since learners make choices that the planners do not expect, it is undoubtedly safer to stick with the targets. But is it wiser?