My unscientific observations about bubbles...
1. In the market, when the top is approaching or has arrived, there is still a rush at "retail" to get in on the previous gains. The online market participation accounts get highly promoted to the masses. Seminars pop up in every large population market showing how to get in on the bonanza. Reality is that the market pros are just looking to broaden the base of unsophisticated buyers during the distribution phase of massive holdings who have reaped the big gains...knowing that they can't all get out the door at the same time w/out new blood.
Shortening of women's skirts also seems to positively correlated.
2. Similar behavior shows up in real estate markets where the "buy and flip a house" dog and pony shows begin to show up in a market. This usually signifies the need for holders of properties to generate buyers in order to get out at or near high points in the cycles. Several areas show the "down" cycles now are shortened in highly desirable areas...and don't seem to bounce/recover in the not so desirable locals.
No links, just long term personal observations...including being fully in the mkt during the days of "irrational exuberance"... and 9/11.