AUM decay
The empirical pattern where funds with very large AUM find it increasingly hard to generate alpha, as large positions create market impact and limit nimble stock selection.
AUM decay describes a well-documented phenomenon: as a mutual fund grows in size, its ability to generate excess returns (alpha) systematically diminishes. The fund that delivered 4% alpha with ₹500 Cr AUM may struggle to beat its benchmark at ₹25,000 Cr. Size becomes the enemy of outperformance.
What it measures
AUM decay tracks how a fund's alpha and quartile rank have evolved as its AUM grew. It is a diagnostic signal rather than a single formula — it asks: "Is this fund's underperformance partially explained by its own success attracting capital?"
Why it happens
Large AUM creates three compounding constraints:
- Market impact: Buying ₹500 Cr of a mid-cap stock moves its price against you before your full order fills. Returns degrade even before fees.
- Diversification forced: To deploy large inflows, a manager must hold more positions. The 30th-best conviction idea is worse than the 10th-best. The portfolio dilutes toward the benchmark.
- Liquidity constraint: In a small-cap strategy, finding ₹2,000 Cr of deployable mid/small cap ideas at reasonable valuations is much harder than finding ₹200 Cr worth.
How MintByte signals it
MintByte tracks two indicators:
AUM vs. Alpha trend: Rolling 3-year alpha plotted alongside AUM growth. If alpha was consistently +3% when AUM was < ₹3,000 Cr and has trended toward +0.5% as AUM crossed ₹20,000 Cr, this is a classic AUM decay signal.
Category AUM percentile: Where this fund's AUM sits within its category. A fund in the top 10% of its category by AUM is more likely to face capacity constraints than a fund in the median.
A rough empirical threshold for Indian markets:
- Large-cap funds: Capacity constraints become visible above ₹30,000–50,000 Cr (market is deep).
- Mid-cap funds: Noticeable above ₹15,000–20,000 Cr.
- Small-cap funds: Even ₹5,000–8,000 Cr can become constraining; several AMCs have hard-closed small-cap schemes at these levels.
How to interpret
- Rising AUM + falling alpha + falling quartile rank: Classic decay. Investigate before increasing allocation.
- Large AUM + stable alpha: Possible if the fund's strategy is genuinely scalable (liquid large-cap) or if the manager actively manages position concentration.
- Recent hard-closure by AMC: A fund that was closed to fresh SIP lump sums at the AMC's initiative is often a positive signal — the manager is protecting existing investors.
Limitations + caveats
AUM decay is a probabilistic pattern, not a deterministic law. Some large-AUM funds maintain strong performance through genuine index-agnostic strategy or by evolving into quasi-passive large-cap allocation. Isolating AUM decay from broader market cycle effects requires multivariate analysis that this indicator approximates but does not fully perform.
Related metrics
- Alpha — the primary output that AUM decay erodes.
- Quartile Rank — peer-relative standing that visually reflects AUM decay effects.
- Expense Ratio Drag — the cost layer on top of AUM decay; both work against net-of-fee returns.
Sources
AUM data: AMFI monthly scheme-level AUM disclosures. Alpha series: AdvisorKhoj API (monthly rolling 3-year). Decay trend computed quarterly; flagged when alpha declines > 150 bps coincide with AUM doubling over any 24-month window.