By Vivianne Rodrigues

NEW YORK (Reuters) - Stock market bulls can be forgiven if they bet hard against Big Brown this Saturday in the fleet-hoofed colt's bid to snatch horse racing's first Triple Crown in three decades.

Why? Stocks tend to slump in years when a horse captures the thoroughbred racing's rarest and most coveted prize.

It is more likely pure coincidence than genuine correlation, but the average return on the Standard & Poor's 500 in Triple Crown winner years is a negative 6.4 percent, according to data compiled by Deutsche Bank.

Just 11 horses have pulled off the feat in the past 140 years of racing. In seven of those years, stocks have fallen, while they've risen in just three. One year, 1919, when Sir Barton captured the prize, predates the index.

More recently, of the last five Triple Crown winners, the market's only positive performance occurred in 1978, when Affirmed became the last horse to accomplish the feat of winning the Kentucky Derby, the Preakness Stakes and the Belmont Stakes. That year, the S&P 500 returned 1 percent, according to Deutsche.

Not even the legendary Secretariat -- who completed the 1973 Triple Crown with a 31-length victory at the Belmont Stakes and established the world record of two minutes and 24 seconds for a mile and a half -- gave stocks a boost. The S&P 500 slid a hefty 17 percent that year.

A defeat on Saturday by the big bay colt would be disappointing to horse race fans across the globe. But history shows that stock bulls may have an extra incentive to place a bet on 7-2 contender Casino Drive or even on Guadalcanal, the 50-1 long shot.

Big Brown stands at a prohibitive 2-5 favorite to win.

Horses that have failed to win the Triple Crown at Belmont after winning the Derby and the Preakness have not disappointed equity investors. The average return during these 20 occurrences has been 10.8 percent and the market has posted positive performances 75 percent of the time, according to the bank's data.

For horses that have won the first two legs of the Triple Crown, the most favorable finishing spot at Belmont for equity investors has been the second place. In those years, markets returned an average of 17.8 percent.

However, as Deutsche Bank pointed out, dedicated horse fans and investors alike will be watching Saturday's race with "unbridled" enthusiasm.

"We will either witness history with the first Triple Crown winner in 30 years, or have another, albeit weak, reason to expect better equity market performance in 2008."

(Reporting by Vivianne Rodrigues; Editing by Tom Hals)