He wonders about what caused stock market capitalization to grow so much relative to GDP starting in the late 1980s.

I think that the answer is simple: The US government started to subsidize stock purchases, specifically the IRA and the 401(K), and it drew more money in to the markets, and that money bid up asset prices.

Of course, I like simple answers, and I am not a stock broker or an economist, so I would appreciate hearing alternate theories.