The "Take home message of the talk" (as she says at 16:47) is that the choice to take risk differs on whether the situation is perceived as a gain or a loss -- regardless of the risk/reward being exactly the same in each case. When presented with the option of either (a) 2 grapes, or (b) 50/50 chances for either 1 or 3 grapes:
- Monkeys take the safe choice (a) in a gain situation, i.e., start with 1 grape and possibly add some more later,
- Monkeys take the risky choice (b) in a loss situation, i.e., start with 3 grapes and possibly take some away.
That being the same as humans tend to do on analogous tests. My personal interpretation is that this points out how negative numbers are actually a very sophisticated, hard thing to deal with for most people (and other organisms). Most of the time in a natural community you'd be taking actions to gain things -- the "loss" scenario is somewhat artificial and abusive, and we're not set up naturally to deal with that well (i.e., we don't have a natural built-in processor for negatives, and for most brains things just kind of go "kablooey" when forced to deal with them).