Sector Diversification and Concentration Risk: How Much of One Stock Is Too Much

A portfolio can hold twenty different stocks and still be dangerously undiversified if eighteen of them are technology companies, or if one position has grown to dominate the account — concentration risk hides inside portfolios that look diversified on the surface.

Two Different Kinds of Concentration

Single-stock concentration happens when one holding — often employer stock from RSUs or options, or a long-held winner that’s compounded for years — grows to represent an outsized share of net worth. Sector concentration is subtler: an S&P 500 index fund is often assumed to be broadly diversified, but the index’s own methodology means its top sector weighting has run well above 30% in recent years as a handful of mega-cap technology companies have grown to dominate the index’s market-cap weighting.

Rules of Thumb Advisors Actually Use

Many wealth managers flag any single stock position exceeding roughly 10–15% of investable assets as a concentration worth actively managing, and get more aggressive about trimming above 20–25%. For sector exposure, the comparison that matters is against a broad benchmark: if a portfolio’s technology weighting is meaningfully above the S&P 500’s own tech weighting, that’s an active bet being made, whether or not it was made on purpose.

What Managing It Actually Looks Like

The tools are straightforward even if the decision is emotionally hard: staged selling over multiple tax years to manage the capital gains hit, exchange funds for concentrated stock (pooling shares with other holders of concentrated positions to diversify without a taxable sale), covered calls to generate income while gradually reducing exposure, and for highly appreciated shares, donating a portion directly to charity to avoid the capital gains tax entirely. Rebalancing toward a target allocation on a schedule, rather than only when a position feels uncomfortably large, is the simplest way to keep concentration from building unnoticed in the first place.

Written by Samuel, Certified Public Accountant

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