Asset allocation and diversification are investment strategies built on the idea that different investments rotate in and out of favor. There’s no sense holding a lagging investment if it’s not eventually going to have its day in the sun.

For some asset classes, this rotation has not happened for a long time.

Last week, the bull market in U.S. stocks became the longest on record, dating back nearly ten years to March of 2009. This means investors have gone a record long period without seeing all the benefits of owning bonds, including how they cushion portfolios on the downside.

Two other trends that have gone on for record long periods are the outperformance of U.S. versus overseas stocks and the outperformance of Growth-style versus Value-style stocks (e.g., high-flying technology stocks versus conservative dividend- paying stocks).

One explanation for why markets haven’t rotated is that the economic expansion has been unusually long and stable (bottom chart). There has been nothing to upset the status quo (as in, a recession) since June of 2009.

This means current leadership trends could persist despite far exceeding historical precedent. On the flipside, the extended nature of the cycle means the rotation between leading and lagging sectors could be significant when it does come.