Monkeys keep turning out to be smarter than people think they are. Researchers have shown that they can count to four and are aware of differences between languages like Dutch and Japanese, even though they don’t known what is being said. Now, Harvard psychologists find that monkeys can draw correct conclusions about novel situations. For example, shown a white towel that turns blue, a blue knife, and a glass of blue paint, they can figure out that the paint, not the knife, is responsible for the change in color.

“Our studies reveal a striking continuity between humans and monkeys in their capacity to draw causal inferences without the help of familiarity with the events or situation,” says Marc Hauser, a Harvard professor of psychology. “This ability highlights the richness of the monkey mind in terms of its understanding of the material world.”

Hauser has been working with a colony of free-ranging rhesus monkeys on an island off Puerto Rico for many years. He and Bailey Spaulding, formerly a student of his, tested individual adult males and females of the colony on their ability to figure out cause and effect in unfamiliar situations.

In their experiments, they used a glass of water and a knife along with a whole apple and an apple cut in half. The knife can halve the apple, but the water can’t. Do the monkeys grasp this?

In one set of tests the monkeys saw a glass of water and two whole apples. Then they viewed a knife being lowered and the apple cut in half. These are two perfectly plausible situations. Next, they saw the glass of water and two halves of an apple. Following this, a knife was lowered, and two apple halves seemingly became a whole apple.

To a human, even an infant who had never seen such things before, the last two apparent happenings would never really happen. Can monkeys infer the same outcomes? Evidently, the answer is “yes.” They looked longer when a glass of water appeared to cut the apple than when a knife seemed to do the same. The longer look signaled disbelief.