A small number of stocks are driving most of the outsized gains in U.S. equities this year.

So is that a problem for investors?

I'm Allison Nathan, and this is Goldman Sachs exchanges.

Each month, I speak with investors, policymakers, and academics about the most pressing market-moving

issues for our top of my report from Goldman Sachs research.

This month, I spoke to two prominent market watchers who are an opposite size of this very

relevant debate. David Koston, our chief U.S. equity strategist in Goldman-S.

research, believes that investors should be concerned about the high level of U.S. equity

market concentration today. That's because he finds that high

concentration is associated with lower returns over the longer run. But Owen Lamont,

senior vice president and portfolio manager at Acadian Asset Management, disagrees. He says that

current concerns about high U.S. market concentration are overblown. I started off by asking my colleague David Koston just how unusual the level of market

concentration is today. He discussed two measures he uses to determine this. The share of total

S&P 500 market cap accounted for by the top 10 stocks and the market cap of the largest stock

relative to the 75th percentile stock.

One measure that's easy for people to comprehend, understand, track, follow, what have you,

is the weight of the S&P 500 market cap that is contributed by the top 10 stocks. We have data going back 45 years, and we

have data on daily basis. You can actually look at it very specifically. And the specific number is

those top 10 companies today comprise around 36% of the market cap. And to go back over time,

it's around 20% at the peak of the dot com boom in 2000. It was about

25%. And then we use another approach or time series that brought the data back 100 years.

Today, 36% of the market cap is the top 10 stocks. Using another metric, we are at a level that we've

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