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Insights

A fractured market: what September revealed about diversification

The headline number from September was almost nothing. The S&P 500 fell roughly half a per cent for the month. On that figure alone, it was a quiet September — a market pausing, perhaps catching its breath.

The headline number was also false.

What the average concealed

Beneath that near-flat index, the typical S&P 500 stock lost nearly six per cent in September. Of the eleven sector groups that make up the index, ten fell — by between half a per cent and more than seven per cent. Only one rose: technology, up just over five per cent, carried narrowly by a handful of chip and AI names. Intel and AMD each rallied around thirty per cent for the month. Microsoft had its best quarter since 1991.

The index held its ground because a small number of very large companies did the work for it. The ten largest companies in the S&P 500 now account for roughly forty per cent of its total weight — a level of concentration without precedent in the index's history, driven almost entirely by the AI-related names that have dominated market gains since late 2022. When those names rise, the headline index rises with them, regardless of what is happening to the other four hundred and ninety constituents.

This is not, in itself, an argument against owning those companies. Many are dominant, profitable businesses. It is an argument about what "S&P 500 exposure" has come to mean. An investor holding a broad index today is holding a portfolio far more concentrated in outcome, and far more dependent on a single technological narrative, than the same index represented a decade ago.

Financials told the opposite story. The sector fell more than six per cent in September — its worst month since March 2023 — as rising bond yields pushed up the cost of capital and pressured demand for loans. Some individual names moved far more sharply: Blackstone fell over twenty per cent for the month; BlackRock fell close to eight per cent. A portfolio holding both technology and financials in September held two assets moving in almost opposite directions for almost opposite reasons. That is what genuine diversification looks like when it is working.

Two markets, one month

The second divergence sat between equities and bonds. On the last day of September, the Fed's preferred inflation gauge — core PCE — came in cooler than expected, rising 3.4 per cent over the year against forecasts of 3.7 per cent. In an ordinary year, cooler-than-expected inflation is unambiguously good news for both stocks and bonds. This time, technology stocks rallied on the news and the broader market fell anyway, while bond yields continued climbing regardless.

The thirty-year US Treasury yield closed September above 5.6 per cent — its highest level since 2002, a stretch of more than two decades. It reached that level on a seven-session losing run for long bonds — the longest such run in two years. The specific data point that should have eased bond-market concern pointed the other way instead. The explanation offered by market participants was not primarily about inflation expectations. It was about supply: the scale of government borrowing, a heavy corporate debt calendar, and a bond market demanding more compensation — a higher "term premium" — for holding long-dated debt in an environment of persistent fiscal deficits.

A third input ran through both markets simultaneously. Oil prices moved on geopolitics rather than on economic data: Brent crude slipped below US$103 a barrel as the United States ordered a further release from its emergency reserves amid a stalled US-Iran negotiation. Energy costs feed directly into inflation readings and, through that channel, into the same bond-yield story driving the thirty-year rate higher.

And geography added a fourth, independent dimension. China's factory activity returned to expansion in September after a two-month contraction, even as the Shanghai Composite fell more than six per cent for the quarter — its steepest quarterly decline since 2022. Whatever was moving US technology stocks, US bond yields and Middle Eastern oil markets in September was not moving Chinese equities in the same direction, or for the same reasons.

The principle

None of this calls for a view on where markets go next. It is, instead, a live illustration of why portfolio construction does not start with a forecast.

Genuine diversification is not a matter of holding more things. It is a matter of holding things exposed to genuinely independent return drivers — across asset class, geography and manager — so that no single narrative, however dominant it has become, determines the whole outcome. September's market held three or four such narratives running in parallel: an AI-driven technology rally that carried the index; a bond market responding to fiscal and supply pressures that most inflation commentary had not anticipated; an energy market moving on geopolitics; and a Chinese market moving on its own domestic cycle. A portfolio built around any single one of these stories — all technology, all US equities, all optimism that falling inflation means falling yields — would have been exposed to a result that single story alone could not explain.

This is also why a structural tilt toward equities is not the same decision as a bet on concentration. Owning productive enterprise at scale remains the foundation of long-horizon wealth. The way that ownership is diversified — across sectors that move on different drivers, across geographies that do not move together, and into real assets that respond to inflation and currency pressure through mechanisms equities do not share — is what determines whether a month like September tests a portfolio or simply moves through it. Drift toward whatever has recently performed, as September's technology concentration illustrates, is exactly the drift considered rebalancing exists to correct.

The lesson of a month like this is not that an investor should have avoided technology, or bonds, or oil-sensitive assets. It is that no single read of the month — "stocks were flat," "inflation eased," "rates are rising" — captures what actually happened. A portfolio constructed for genuine diversification does not need to resolve that complexity. It is built to hold through it.

If you would like to review how exposed your portfolio is to a single market narrative — rather than how many holdings it contains — we welcome the conversation.

Ben Wieland
Partner, Wealth Manager

1300 102 542 | 0423 710 820
ben@egu.au

Sources

This is general advice. It does not take account of your objectives, financial situation, or needs, and is not a substitute for advice that does. Before acting on anything in it, consider whether it suits your circumstances, and consider the relevant Product Disclosure Statement.

Ben Wieland