An average human offered the choice to accept either $500 or a coin flip for $1,000 will choose the safe bet -- $500 guaranteed. This is a rational choice, and avoids taking risk without a premium (reward.)
An average human will make the opposite choice if offered the opposite decision. Would you rather have $500 confiscated from you, or flip a coin? Heads you lose $1,000 and tails you lose nothing. Most people choose to flip the coin.
Turns out that the average monkey will do the same thing if offered similar choices with tokens and food, as shown in the video above, where Laurie Santos rambles at length about how irrational human beings tend to be when it comes to financial risk-taking, and endlessly implies that some sort of oversight is needed to correct genetic defects in our mental design. Dan Ariely wrote an entire book about it, and also appears at the TED conference on a regular basis to talk about why regulation is needed to force us to make rational decisions. Myriad progressive thinkers have used examples like this to prove that we don't know what is best for our own lives.
A better question, I think, is why? Why do we make this seemingly irrational decision to avoid risk if offered additional food, and to take more risk if threatened with loss? I think I have the answer. For most of our evolution, we have existed at the fringe of starvation, disease, and death. Life has historically been a constant and never-ending struggle for survival. Losing what we already own would be potentially fatal. If viewed in this context, our intuition regarding risk-taking proves to be correct.
Imagine you have worked tirelessly all summer to harvest and preserve enough meat and root veggies to barely last you through the winter, and you are offered a choice. Take enough additional food to guarantee your survival, or gamble to win more than you need to make it through the cold months. The choice is the same as above -- you would choose the safe bet and guarantee your passage through the winter.
And if offered the opposite choice -- to flip a coin and risk losing all of your food or none of your food -- or to choose losing half of your food -- the choice is also easy. Because you cannot afford to lose any food without starving to death, you don't care how much food you lose. Either way you're going to die, so your only choice is to gamble and hope that the coin comes up in your favor, allowing you to keep all of your food.
This type of decision making isn't irrational at all if the gambler cannot afford to lose, and doesn't stand to change his life very much by gambling for more.