Pessimism is deeply rooted in our biological makeup. It's also one of the reasons many people don't live up to their potential.

It turns out that our brains weren't designed for the modern world. Instead, they were programmed to maximize our chance of survival during the Stone Age, a span of 3.4 million years during which an estimated one in eight males died from violent conflict.

As a consequence, explains Rick Hanson in Hardwiring Happiness, our brains evolved a "negativity bias" that looks for bad news, reacts intensely to it, and quickly stores the experience in the hippocampus region of our brain.

Pessimism: Once an asset is now a liability
It goes without saying that a pessimistic view of the world served the human race well over the past few million years. Without such a perspective, it's easy to conclude that our ancient ancestors would have been eaten by saber-toothed tigers long ago.

But the reality is that we no longer live in the Stone Age. And with the dawning of modern society, the value of pessimism has receded -- completely. To succeed in today's world, it's essentially necessary to be an optimist.

This new reality follows from a simple fact. That is, psychologists have demonstrated that pessimists not only overestimate threats, but they also underestimate opportunities.

"As a result," Hanson wrote, "we end up preoccupied by threats that are actually smaller or more manageable than we'd feared, while overlooking opportunities that are actually greater than we'd hoped for."

I've been reminded of this over the past few weeks while reading through the autobiographies of entrepreneurs such as Ray Kroc, Sam Walton, and Howard Schultz -- the driving forces behind McDonald's (NYSE: MCD), Wal-Mart (NYSE: WMT), and Starbucks (NASDAQ: SBUX), respectively.

All three of these men exuded optimism. "There's almost nothing you can't accomplish if you set your mind to it," Kroc wrote. "Most people can achieve beyond their dreams if they insist on it," says Schultz. And Walton went so far as to say that he never even considered losing. "Thinking like that often seems to turn into a sort of self-fulfilling prophecy," the Arkansas-born merchant observed.

Not coincidentally, each of these men had a peculiar knack for both recognizing and pursuing opportunities. Kroc and Schultz are particularly emblematic of this, as neither of them actually founded the chains that their names are now associated with -- that is, McDonald's and Starbucks. They were simply the first to grasp the potential.

Kroc happened upon the McDonald's concept six years after the McDonald brothers opened their hamburger stand in 1948. And Schultz didn't walk into his first Starbucks until the Seattle-based chain had been in operation for more than a decade.

The point is that optimism and opportunity go hand in hand. Without the former, you have little hope of recognizing and genuinely appreciating the latter.

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John Maxfield has no position in any stocks mentioned. The Motley Fool recommends and owns shares of McDonald's and Starbucks. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.