"A conservatism bias means that investors are too slow (too conservative) in updating their beliefs in response to recent evidence. This means that they might initially underreact to news about a firm, so that prices will fully reflect new information only gradually. Such a bias would give rise to momentum in stock market returns."
Bodie, Kane and Marcus (2005)
"This is the tendency to cling tenaciously to a view or a forecast. Once a position has been stated most people find it very hard to move away from that view. When movement does occur it is only very slow (this creates under-reaction to events).
Montier (2002) [book]
To make matters worse, once people have formed a probability estimate, they are often quite slow to change the estimate when presented with new information. This slowness to revise prior probability estimates is known as “conservatism” (Phillips & Edwards, 1966). Conservatism is the tendency to change previous probability estimates more slowly than warranted by new data. Most often, slowness is defined relative to the amount of change prescribed by normative rules such as Bayes’ theorem (Hogarth, 1975)."
Plous (1993), page 138
"...conservatism as a cause of post-earnings-announcement drift..."
Shefrib [book]
"A combination of overconfidence, together with anchoring-and-adjustment leads investors and analysts to adapt insufficiently to the arrival of new information. The result is conservatism."
Shefrin [book]
"According to a psychological principle of conservatism, people are slow to change their opinions."
Shiller (2000)
HELBOK, G. and M. WALKER, 2004. On the nature and rationality of analysts' forecasts under earnings conservatism. The British Accounting Review. [Cited by 4]