In the United States, there were two key decisions. The first, in the 1970’s, deregulated commissions paid to stockbrokers. The second, in the 1990’s, removed the Glass-Steagall Act’s restrictions on mixing commercial and investment banking.
Of equal importance were the rise of China and the decline of investment in Asia following the 1997-1998 financial crisis. With China saving nearly 50% of its GNP, all that money had to go somewhere. Much of it went into US treasuries and the obligations of Fannie Mae and Freddie Mac. This propped up the dollar and reduced the cost of borrowing for US households, encouraging them to live beyond their means. It also created a more buoyant market for the securities of Freddie and Fannie, feeding the originate-and-distribute machine.
Again, these were not outright policy mistakes. Lifting a billion Chinese out of poverty is arguably the single most important event in our lifetimes. The fact that the Fed responded quickly prevented the 2001 recession from worsening. But there were unintended consequences. The failure of US regulators to tighten capital and lending standards when abundant capital inflows combined with loose Fed policies ignited a furious credit boom. The failure of China to move more quickly to encourage higher domestic spending commensurate with its higher incomes added fuel to the fire.