This usage contrasts with models in the genre pioneered by Bernanke and Gertler (1989), where time-variation in collateral values can lead to large changes in the supply of certain types of credit but where expected returns on credit are constant over time. 3  Habit-formation models have this property. See, for example, Campbell and Cochrane (1999). 4  See Gennaioli, Shleifer, and Vishny (2012).
5  For example, Griffin and others (2011) document that during the run-up of the Internet bubble, from January 1997 to March 2000, individual investors were substantial net buyers of tech stocks, via both direct purchases as well as investments in tech-sector-specific mutual funds.