Glide Path vs. Allocation Drift: What's the Difference in a Fund's Asset Mix?
Check a fund’s stock-to-bond split today and again in a few years, and it will almost certainly look different. Whether that change was intentional or just something that happened on its own is a distinction worth untangling.
The short answer
A glide path is a deliberate, pre-scheduled shift in a fund’s asset mix, typically moving from more stocks toward more bonds as a target date approaches. Allocation drift is different: it’s an unintentional change in that same mix, caused simply by one asset class growing faster than another and gradually taking up a larger share of the portfolio. One is designed into the fund on purpose; the other happens as a side effect of markets moving, unless something corrects for it.
How a glide path works
Funds built around a glide path, most commonly target-date funds, have a schedule baked into their design. Early on, the fund might hold a larger share of stocks, aiming for growth over a long time horizon. As the target date gets closer, the fund’s managers gradually shift the mix toward more bonds and cash-like holdings, aiming to reduce volatility as the time to use the money approaches. This shift is scheduled and rules-based, decided in advance rather than reacting to what the market happens to be doing in any given year. For a deeper look at how this specific mechanism works, see what glide path means in a target-date retirement fund.
How allocation drift happens
Allocation drift has nothing to do with a schedule and everything to do with math. If a portfolio starts with, hypothetically, 60 percent in stocks and 40 percent in bonds, and stocks then grow faster than bonds over some stretch of time, that ratio doesn’t stay put on its own — the stock portion mechanically becomes a larger share of the total simply because it grew more. Nobody made an active decision to increase stock exposure; it happened because the two asset classes performed differently, and the original mix was never rebalanced back toward its starting point.
Why the distinction matters
Confusing the two can lead to a false sense of what’s driving a portfolio’s current mix. A shift caused by drift isn’t a deliberate risk decision — it’s often the opposite of what an investor intended, since it tends to push a portfolio toward whatever has been performing best recently, which can quietly increase risk exposure at exactly the wrong time, right after a run-up. A glide path shift, by contrast, is intentional and typically moves in the reduce-risk direction as a target date nears, regardless of which asset class has recently outperformed.
How each one gets addressed
A glide path is maintained by the fund’s own management process — an investor holding a target-date fund generally doesn’t need to do anything for the scheduled shift to happen, since it’s part of the fund’s design. Allocation drift, on the other hand, only gets corrected through portfolio rebalancing — periodically selling some of what’s grown and buying more of what hasn’t, to bring the mix back toward a chosen target. Some funds rebalance on a set schedule internally; a portfolio built from separate funds outside of that structure relies on the investor, or an advisor, to do this rebalancing.
What to weigh
Understanding whether a change in a fund’s asset mix is scheduled or drifted matters for figuring out whether the current mix still reflects an actual goal. A glide path shift is working as designed. A drifted mix is simply the byproduct of unequal growth, and left unaddressed for long enough, it can leave a portfolio holding a very different asset allocation than what was originally intended, without anyone having decided that on purpose.
The takeaway
Not every change in a portfolio’s stock-to-bond mix means the same thing. One kind is a plan unfolding on schedule; the other is drift that quietly happens in the background unless something checks it periodically and brings it back in line.