Quantitative strategy guru James O’Shaughnessy says history indicates that the rest of this decade should at least be a decent one for stocks, and that the 20 years after that should be particularly good. O’Shaughnessy tells Yahoo! Finance’s Breakout that his firm studied market returns over 20-year periods and found that when stocks have outperformed for a 20-year span — as they did in the 1980s and 1990s — the following 20-year span tends to feature poor returns. That might seem like bad news for the rest of this decade, given that we’re in the middle of a 20-year period that followed a strong 20-year period, but it’s not. “What we didn’t count on was that we’d have a decade with two severe bear markets in it,” O’Shaughnessy explains. Because of that, the poor performance for the current 20-year period may well have been front-loaded. That means it will take decent returns for the rest of this decade just to make the 2000-2020 period’s returns in line with some of the worst in history. And then after that, the next 20 years should be quite good, he predicts.