Kevin Warsh makes his debut at the helm of the Federal Reserve on Wednesday, as the central bank wraps a policy meeting with markets broadly expecting rates to stay on hold at 3.50%-3.75%.

While all eyes are on what Warsh says in his first press conference — and on how Fed officials reshape the dot plot against a backdrop of stickier inflation and the war in Iran — there's another question worth asking: How has the S&P 500 historically performed once a new Fed Chair takes over?

The last seven leadership handovers since 1970 are shown below, each paired with the market's forward 1-month, 3-month, 6-month and 12-month returns.

One Month Later: A Reliable Honeymoon

In the first month after a new chair takes office, the S&P 500 — tracked by the SPDR S&P 500 ETF Trust (SPY) — has returned an average of 2.3%, posting gains in six of seven episodes.

The best showing came under Janet Yellen, with the index up 6.0% in the month after her February 2014 start. Yellen inherited a maturing bull market and a strengthening recovery, and her dovish, continuity-first message reassured investors that the Fed would taper its bond buying only gradually while keeping rates pinned near zero — removing a major source of uncertainty.

The lone decline belonged to Alan Greenspan, down 3.4%. Greenspan was simply unlucky with timing.

Three Months Later: The Greenspan Distortion

Three months out, only four of seven episodes were positive, dragging the average to a negative 1.5%.

That figure is heavily skewed by the 27.4% collapse that followed Greenspan's start, as the market endured the October 1987 crash — a selloff driven by stretched valuations, portfolio insurance and program trading far more than by monetary policy.

In other words, it was less a Fed event than a market accident.

Six Months Later: Recovery Reasserts Itself

Six months on, four episodes were again positive, with an average return of 2.5%.

Greenspan's era kept the worst outcome at -23.2%, while the strongest belonged to G. William Miller, with the S&P 500 up 21.6% half a year after his March 1978 arrival.

One Year Later: Green Dominates

A year out, the greens outweigh the reds by six to one. The only negative came under Greenspan, with the S&P 500 still more than 20% below its level a year after he took over in August 1987. The best 12-month return again went to Yellen, with the index up 17.7%.

Notably, the last three Fed Chairs — Ben Bernanke, Yellen and Jerome Powell — all presided over a higher S&P 500 one year into their tenure.

The Bottom Line

Strip out the 1987 crash, which had little to do with who was running the Fed, and the pattern is clear: a new Fed Chair has historically been more friend than foe to the stock market.

For Warsh, history sets a constructive — if not guaranteed — starting point.