Risk tolerance doesn't change much over time.

. . . [F]inancial risk tolerance is a genetic and predispositional stable personality trait and is unlikely to change over the life of an individual.

That is form the new paper "A Longitudinal Study of Financial Risk Tolerance" by Van de Venter and Michayluk. It is forthcoming in the Journal of Economic Psychology (August 2012). A draft is here.  

Another new paper, "Individual Financial Risk Tolerance and the Global Financial Crisis" by Gerrans, Faff, and Hartnett, proposes a similar explanation:

This paper examines the change in financial risk tolerance associated with the global financial crisis of 2007-2009 using test and retest data from investors who use the FinaMetrica risk tolerance survey and after controlling for demographic and regional variations. In absolute terms the change in risk tolerance is low and contrasts with a prevailing view that risk tolerance is an elastic psychological state overly influenced by the pervading market conditions. While risk tolerance measured from retests conducted through the second half of 2008 and first half of 2009 was lower than that indicated for retests conducted prior to the global financial crisis, the change was no greater than a few measurement points. The results suggest that while risk-taking behaviour was impacted greatly by the global financial crisis, change in investor financial risk tolerance per se appears an unlikely driver of such a shift in behaviour, with other factors such as revised risk and return perceptions and expectations potentially playing a more pivotal role.