Primates other than chimpanzees also display highly intelligent behavior. There is much evidence suggesting that the origins of personal intelligence can be found in species other than our own. In monkeys, for instance, there is normally an attachment link between mother and infant similar to that seen in psychologically healthy humans. Harry Harlow's research pertaining to motherless monkeys has determined the presence of a similar psychopathological state like that seen in humans under similar circumstances. A monkey raised without a mother develops abnormal affective behavior. They are not able to react appropriately to other monkeys and cannot assume their proper role in the dominance hierarchies. Motherless monkeys will cower in fear or aggressively attack other monkeys in inappropriate situations. They also prove incapable of raising their own offspring.¹⁹...A female motherless monkey does not make a good mother, which indicates that motherhood is not completely instinctual.

Ronald Myers and his colleagues at the National Institute of Health have found that they can produce abnormal social reactions in monkeys by certain types of surgical intervention. For instance, by removing the prefrontal cortex in juvenile primates they produced a decrease in the use of facial and vocal communication, an alteration in aggressiveness and patterns of grooming, a decreased participation in play activity, as well as frequent sessions of hyperactivity of an aimless variety.²⁰

The hundredth-monkey phenomenon, as described by Lyall Watson in his book Lifetide, is also very revealing relative to intelligence in animals, although he has since indicated that he regards it as at least partly metaphorical. He tells of a tribe of monkeys on an island close to Japan to which was introduced a new food, freshly dug dirt-covered sweet potatoes. The monkeys were reluctant to eat them because they were dirty, and it wasn't until an 18-month-old female monkey named Imo solved the problem by taking the potatoes to a nearby stream and washing them that progress was made. Imo taught this new behavior to her mother and other younger monkeys, who also in turn taught their mothers. The mature adults did not adopt the behavior unless directly taught by their children. Then rather suddenly it became a universal phenomenon, as if a threshold was passed that required a critical mass. Not only were all the monkeys on the one island suddenly washing and eating potatoes, but this behavior seemed to jump natural barriers to other islands, as well as to the mainland of Japan so that other tribes of monkeys were copying this behavior. This represents an abstraction of thought and cultural breakthrough. Even more importantly, it suggests that there may be other yet-to-be-recognized forces operational in the process of evolution.²¹

These same Macaque monkeys that have traditionally loathed water now have learned to enjoy it. They first discovered that the salt from washing potatoes in sea water improved the potatoes' taste. They later found that scattered food grains could be salvaged by putting them in water and collecting them as they float, allowing dirt and sand to separate from them by sinking. These two discoveries produced profound cultural changes since the use of water led to swimming. The younger monkeys also found the water to be a delightful playground.²²

There is also experimental evidence that monkeys are capable of abstract thinking. According to Griffin, monkeys were able to consistently select from among three objects that single item which was unlike the other two. He concluded that this seemed to indicate they were able to grasp the abstract concept of dissimilarity.²³

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