A recent study has pointed out that chimpanzees and orangutans have personalities similar to humans.
The new findings address a long-standing debate about whether great apes possess human-like personalities or if such perceived behaviour is an anthropomorphic projection of human observers, the researchers said.
The research team used a statistical technique to "remove" any biases apparent in human observers of the apes' behaviour, and they say their findings suggest man and ape really do share "personality dimensions".
"[Chimpanzees] have the same social problems that we do, they want to make friends and find mates and sort of gain position within their society," the BBC quoted team member Mark Adams, a researcher who conducted the research while studying for his PhD at the University of Edinburgh, UK, as saying.
Dr Alexander Weiss, senior lecturer at the University of Edinburgh, who also worked on the study, agrees that chimpanzee personality is "highly similar" to that of humans.
Researchers categorise human personality into five "dimensions", sometimes known as "the big five", he explained.
"Those dimensions are neuroticism, extroversion, openness to experience, agreeableness and conscientiousness," he said.
Previous studies into non-human primates suggest that chimpanzees (Pan troglodytes) share these five dimensions with people, whilst orangutans (Pongo pygmaeus and Pongo abelii) display three of the five: extraversion, neuroticism and agreeableness.
These shared personality dimensions are best explained by our genetic similarities, according to Dr Weiss.
"Humans and chimps share a common ancestor about 4 to 6 million years ago," Weiss said.
According to Dr Weiss, the common ancestor for humans and orangutans is thought to have existed fifteen million years ago, which explains why chimpanzees and humans are more similar in personality than orangutans and humans.
There is continuing debate amongst experts as to whether scientists should use anthropomorphic projections while studying how animals behave.
Dr Clive Wynne, professor of psychology at the University of Florida, Gainesville, US describes anthropomorphism as "a mistake" when "trying to understand animal behaviour."
"Human beings have a very natural tendency to project human agency into almost anything that moves," he said.
"It's very deeply ingrained into our ways of trying to understand the world around us," he said.
But despite our inevitable "human perspective" in the way we see the animal kingdom, he says, "since these animals are not us, although it is difficult, we should nonetheless struggle to get our own perspective out of the way and to try and see them for themselves."
The research team carrying out the study wanted to test the extent to which human observers of chimpanzee and orangutan behaviour might be biased in their reports.
"There's sort of a fear that human observers and 'raters' are projecting their own ideas about personality on to these animals," Adams said.
But until now, this theory "hasn't actually really been tested in great apes."
Members of the research team - who also came from Kyoto University in Japan and the University of Arizona, Tucson, US - issued questionnaires to around 230 people observing chimpanzees and orangutans in zoos and research centres in the US, Canada, Australia and Japan.
The survey described about 40 to 50 personality "items", which when grouped together make personality dimensions.
The human observers - called "raters" - were instructed to rate the apes' behaviour on a one-to-seven point scale for each personality item.
From the questionnaire results, the team determined the type of biases present in the raters' observations of the animals.
"We used a statistical technique to remove these observer differences before extracting personality traits from the data," Adams explained.
"What we found is that controlling for these differences among observers made no difference, which suggests that the observers are not projecting their own ideas about personality onto the animals," he said.
Dr Weiss says that the research "vindicates both the view that chimpanzees have personalities and perhaps the more controversial statement that their personalities are quite similar to those of humans."
The study features in the journal Animal Behaviour.