Another instalment in a series of articles detailing how to design a secure, income-producing portfolio.

There are around 72 million baby boomers in the U.S. and they’re an affluent group. They account for less than a quarter of the U.S. population but half of net household wealth, as of 2015, according to Deloitte. Much of that wealth is held in IRAs, 401(k)s, or other investment vehicles; in other words, most of their wealth is in stocks. As they retire, currently at a rate of 10,000 a day, a lot of these boomers are planning to sell stocks to generate cash to live on. That means that this enormous swathe of the population has purchased securities on the assumption that, at the very moment they will all need liquidity, markets will be sufficiently bullish to support them all selling at more or less the same time. That’s market timing on a grand scale. A lot of people are making a big bet, with their retirement security hanging in the balance.

There are already reasons to think that this bet won’t go as well for them as they’re hoping. We know that the decades during which boomers poured the greatest volume of savings into liquid securities also coincided with a period of high stock valuations. This makes intuitive sense as a simple matter of supply and demand. The baby boomers, who are the largest generation in U.S. history, demanded securities in great volume, to serve as their savings vehicle; their demand is essentially the aggregate of millions of those individual financial planning meetings. Their collective hunger for stocks would naturally push prices up, and price data support this idea.

Will U.S. stock markets experience a steep decline and a 20-year bear market when the bulk of the boomers have cashed out? No one knows. Market timing is a bad idea because we don’t know how markets are going to behave in the future, and if we project that they’re going to be bearish we can be no more certain than if we’re projecting bullish. At a minimum, though, we should not merely presume that markets will accommodate our retirement.

Plenty of people argue that the boomers’ retirement will not inflict great pain on markets. The most common defence, and one often made by large financial institutions, is that markets are highly efficient at pricing in widely known events. According to this view, everyone knows the boomers are going to retire and sell a lot of their stocks and thus it’s already priced into today’s valuations.