I has certainly moved. If you add to a sample (marketcap values) values that are predominantly large (1982-1999) --- more precisely greater than the mean --- the new mean will be larger.
"What has changed between ‘82 and now? You are essentially asking the question, what makes bull markets?
"Candidate ideas: 1) the Reagan tax cuts; "
That certainly had a role from 1982 to mid 1980s.
"2) the increased productivity due to PC’s;"
At the time, this was one of the factors credited with the 1990s boom.
"3) the beginning of IRA’s (increased demand for market securities);"
You are probably correct here, too. Whether its effect was large is unclear to me. What funds paid for the IRAs? If people purchased securities with cash they would otherwise invest in mutual fund, then the effect is zero, since the funds would invest in securities anyway. If they bought securities with funds that they would otherwise spend on consumption goods, then the effect should be there. I just don't know data to have any intuition here.
Another difficult question is, of course, whether your list is exhaustive. As one identifies smaller factors, it becomes progressively difficult to identify their respective effects.
You said” I has certainly moved. If you add to a sample (marketcap values) values that are predominantly large (1982-1999) -— more precisely greater than the mean -— the new mean will be larger.
“What has changed between ‘82 and now? You are essentially asking the question, what makes bull markets?”
Actually no. What I really mean, has the mean ratio permanently moved from 60% to 80-100%, through bull and bear markets? Is the time period from 1980-2010 merely an aberration that will return to the 60% mean, or has there been a permanent change in our nation's financial structure?
The Reagan tax cut effects continued until the 1990 budget when taxes were increased, causing the recession. Less obvious effects were deregulation of energy markets, which lowered the price of oil.
The IRA law passed in the 80’s definitely moved money from consumption to savings in bonds and stocks. For the first time, middle class people had a tax shelter for their income. This created an increased demand in stocks. This was extensively commented upon throughout the 90’s.
Perhaps the greatest act that President Clinton did was the 1994 NAFTA agreement which caused booms in the US, Mexico, and Canada. That agreement negated the effect of his tax increase.
Combined with the 1994 Republican takeover of Congress and a reduction in Federal spending, the 90’s were good for business and labor.