UBS Warns: U.S. Stocks May Face Historic Volatility Ahead of Midterm Elections
nashnova research
UBS chief economist Arend Kapteyn warns that August through October is the most volatile seasonal window in nearly a century of U.S. equity data — and the 2026 midterms will push uncertainty to historical norms or higher, though post-election history points to a sharp rebound.
Why call this "the calm before the storm"?
Kapteyn cites data going back to 1928: August through October is consistently the highest-volatility quarter of the year.
This means → election or not, these three months are when markets get roughest; layering midterm uncertainty on top only amplifies the effect.
With Senate control odds still near 50-50 on betting markets, he sees no reason to expect this cycle's uncertainty — or volatility — to land below historical averages.
What does history say about midterm outcomes?
Since 1950, the president's party has lost an average of 25 House seats and 3 Senate seats in midterm elections.
JPMorgan's Andrew Tyler narrows it further: across 23 midterm cycles since 1934, the ruling party lost roughly 27 House seats and 3 Senate seats on average.
In plain terms = apply those numbers to the current Republican majority — 218 House seats, a 53-47 Senate — and the historical template points to Democrats retaking the House while Republicans narrowly hold the Senate.
Betting platform Kalshi prices a Democratic House takeover at roughly 85%; Republican Senate retention at just 53%.
What happens if Democrats sweep both chambers?
Bank of America's Michael Hartnett expects a sharp sell-off if Democrats win both the House and Senate.
This means → traders' core fear is not a change of party but the policy signals Democrats have already sent: a potential moratorium on data centers and tighter regulation.
This reflects a market pricing AI-related assets as the most exposed sector — a data-center ban would hit today's most crowded trade head-on.
What does the S&P 500's historical path look like?
Kapteyn's historical review: the S&P 500 typically declines from late August through early October in midterm years, then posts an average cumulative return of roughly 14% by the following March (median 16.4%).
In plain terms = the pattern is drop-then-rally — three months of pain before the election, then a strong rebound over the next six months, delivering nearly three times the return of the same period in non-election years (under 5%).
Only three exceptions in the record: the 1978 inflation shock, the 2002 tech-bubble collapse, and the 2018 trade-war-plus-Fed-tightening episode. This reflects a rule that only macro-level shocks can override the post-election rebound pattern.
Can the current calm last?
Kapteyn's conclusion is blunt: with Senate odds at 50-50, "there is no reason to believe this year's uncertainty will be below historical midterm levels."
This means → two variables will decide the trajectory: the evolving election outlook and how markets reprice policy risk.
Put simply = today's low volatility is not "all clear" — it is the market not yet digesting election risk in earnest. History says that once it starts, the turbulence arrives fast.
市场有风险，内容仅供研究参考，不构成投资建议。