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Thank you for your patience in clarifying that. Now I have to rethink my understanding of that chart. What it says is that market capitalization dramatically increased from ‘82 until the collapse of the dot com bubble in 2000. It still has not returned to its mean of 60%. The question I have is, has the mean moved? What has changed between ‘82 and now?

Candidate ideas: 1) the Reagan tax cuts; 2) the increased productivity due to PC’s; 3) the beginning of IRA’s (increased demand for market securities);

Any ideas?

" The question I have is, has the mean moved?"

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.