Investments, tenth edition

12.3 

  Bubbles and Behavioral Economics 

 In Example 12.2 above, we pointed out that the stock market run-up of the late 1990s, 

and even more spectacularly, the run-up of the technology-heavy NASDAQ market, seems 

in retrospect to have been an obvious bubble. In a 6-year period beginning in 1995, the 

NASDAQ index increased by a factor of more than 6. Former Fed Chairman Alan Green-

span famously characterized the dot-com boom as an example of “irrational exuberance,” 

and his assessment turned out to be correct: by October 2002, the index fell to less than 

one-fourth the peak value it had reached only 2½ years earlier. This episode seems to be 

a case in point for advocates of the behavioral school, exemplifying a market moved by 

irrational investor sentiment. Moreover, in accord with behavioral patterns, as the dot-com 

boom developed, it seemed to feed on itself, with investors increasingly confident of their 

investment prowess (overconfidence bias) and apparently willing to extrapolate short-term 

patterns into the distant future (representativeness bias). 

 Only 5 years later, another bubble, this time in housing prices, was under way. As in 

the dot-com bubble, prospects of further price increases fueled speculative demand by 

purchasers. Shortly thereafter, of course, housing prices stalled and then fell. The bursting 

bubble set off the worst financial crisis in 75 years. 

 Bubbles are a lot easier to identify as such once they are over. While they are going on, 

it is not as clear that prices are irrationally exuberant and, indeed, many financial commen-

tators during the dot-com bubble justified the boom as consistent with glowing forecasts 

for the “new economy.” A simple example shows how hard it can be to tie down the fair 

value of stock investments.  

21

    

 In 2000, near the peak of the dot-com boom, the dividends paid by the firms included in 

the S&P 500 totaled $154.6 million. If the discount rate for the index was 9.2% and the 

expected dividend growth rate was 8%, the value of these shares according to the con-

stant-growth dividend discount model (see Chapter 18 for more on this model) would be

   Value

5

Dividend

Discount rate

2 Growth rate

5

$154.6

.092

2 .08

5 $12,883 million