Many people check out the bookcases, Facebook profiles or music collections of potential partners, and some may even sneak a glance into the bathroom cupboard. But what is it like to be a professional academic snoop? Can you tell whether people are attractive or nervous from clues they leave in their bedrooms? Is there a sharp divide between people who do and people who don't carry spare stamps with them at all times? And what do we reveal about ourselves by the individual ways we arrange our towels, stock our fridges, shake hands, walk or even jump?

Sam Gosling, associate professor of psychology at the University of Texas at Austin, knows about all these things, has carried out research on many of them and now shares his findings in Snoop: What Your Stuff Says about You. He has also explored the nature of personality in animals through work on spotted hyenas, chimpanzees, squid and the dogs used by the US Air Force. Both are fairly unusual research topics, and even more unconventional when he started to look into them. So how has he got to where he is today?

Although he is happy to talk about his life and work, Gosling starts off with a striking cautionary note. When he was first a graduate student at the University of California, Berkeley, he had attended a seminar with the celebrated cognitive psychologist Eleanor Rosch. "She handed round a hat with papers saying 'Journalist', 'Soldier', 'Exotic Dancer', 'Baker', 'Architect'," he explains. "Everyone pulled one out and then, using real events from their lives, without making anything up, had to tell a story, provide an explanation of (how they came to be) whatever was written on the paper. And 60 to 70 per cent of the class managed to do it."

The very fact that "you can tell a plausible causal story of how you became something you're not", he says, "undermines the story of how you became what you are". Gosling has, for example, two different, perfectly credible but incompatible accounts of his interest in the personality of animals. Like the rooms we live in, the stories we tell about ourselves do several different things at the same time. They reveal who we really are, how we'd like (and need) to think of ourselves and how we'd like to be seen by others. It is part of the work of the snooping psychologist to separate these different strands.

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Having undermined his narrative in advance, Gosling discusses his life. He was born in 1968, "the heyday of swinging London", and grew up in Hampstead, the son of a psychoanalyst and a writer. By the time he was ten or so, his father was in his late fifties and "no longer able to capture the little whispers of huge Freudian significance", so the family moved out to "a smallholding with ducks and geese and sheep in Gloucestershire".

Here is the first possible story that Gosling says is often imposed on him when people hear he spent his teenage years on a farm. "So you were always familiar with animals and noticed the differences?" they ask. To which it is all too easy to reply: "Yes, I did hang out with sheep and stuff and probably noticed the individual differences, and that primed me to start thinking about personality in animals."

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The most striking moment of self-revelation in Snoop comes at the very end, when Gosling is describing an architect who tries to create perfect homes for his clients by giving them detailed questionnaires about their childhoods, their treasured possessions, where they seek privacy - and even the gripes they have about their partners. This sets him thinking about the "perfectly regimented" and constantly replenished beverages on the shelves of his fridge. What do they express about his own values, fantasies and ideals?

When he was growing up, he realised, his parents were still haunted by memories of wartime rationing - and his father would dilute the ketchup with milk to "extract every last dreg of the precious tomato-flavoured gold". Such austerity had left Gosling with a "deep-rooted need for abundance". Perhaps inevitably, then, he did what people in search of abundance have been doing since the time of the gold rush. After finishing his degree in philosophy and psychology at the University of Leeds, he set out for California.

This was initially on a year's exchange scholarship at the University of California, Berkeley. Then he took a year out, mainly hitching around Africa, before returning to Berkeley to complete his PhD. He moved to Austin in 1999.

It was while he was still in California that Gosling began to investigate the topics he has made his own. He was influenced by one of his advisers, Ken Craik, who "was frustrated that so much research was done inside laboratories filling out surveys. He was one of the founders of environmental psychology. He had been pushing the idea that we should study what people really do, not what they say they do - their behaviour, their acts."

The great achievement of recent work on personality has been the development of the "Big Five" or OCEAN dimensions, which enable one to classify people for their levels of openness, conscientiousness, extroversion, agreeableness and neuroticism (as well as the separate facets that make up these broad traits). Gosling has no doubt that creating such a "common currency of personality" is invaluable for further research and that the Big Five "offer a very useful and true first read on people's tendencies. If I wanted to hire somebody as a salesperson or a receptionist or a truck driver, maybe even a housemaid, I think knowing their Big Five profile would be more useful than knowing their life story. But it's not like really knowing them or knowing them well enough so you could decide whether to marry them."

But just because we now have ways we can measure and discuss personality rigorously does not mean we really know what personality is. Gosling's research addresses some fierce debates in academic psychology, and makes them lively and accessible to general readers in Snoop. One of the key problems is that establishing personality on a scientific basis relies largely on getting people to fill in questionnaires. This not only cuts off the subject from everyday behaviour but allows critics to argue "either, radically, that personality doesn't exist or, less radically, that it's not important". Knowing how "open" a person is will seldom predict how they are going to behave on a particular occasion. Are our actions determined mainly by our underlying traits or the situations we find ourselves in?

There are also questions about how we can assess people's personalities without using highly artificial lab-based methods. Meeting and talking to someone is a pretty good way of telling how extrovert they are, but what about the other dimensions? One of Snoop's major contentions, illustrated with dozens of amusing examples, is that "snooping techniques are the best methods for assessing some of the dimensions - provided you define snooping pretty broadly". And since amateur snoopers make some fairly predictable mistakes, Gosling offers some expert guidance on developing super-snooper skills.

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He notes the subtle differences between a genuinely tidy person's room and a hastily tidied room. He explains the distinctions between several types of hoarder. He argues why some objects are best seen as "social snacks" or "visual self-medication". And he assesses the hidden messages of posters setting out "identity claims" ("Be your own goddess"). Our offices and bedrooms inevitably contain residues of things we have done, from muddy boots and parking tickets to holiday photos. So, however hard we try, we can never hope to fool the alert psychological detective.

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Scavengers who sift through celebrity garbage can make a good living selling stories to the tabloids. One, waxing philosophical, offered the reflection that "garbage is a window into the soul". A colleague of Gosling's has acquired the nickname of "the Indiana Jones of solid waste". So how far is snooping - and, by extension, much academic research - motivated by nosiness and voyeurism?

Gosling disputes the voyeurism suggestion, arguing that "a lot of work in psychology is less research than 'me-search'. People are fascinated by themselves, and it's uncanny how often their own 'issues' relate to the topic they study."

Which leads us back to his work with animals. Perhaps even more than snooping, this had attracted a lot of hostility. "Some people thought it was plain goofy or frivolous," he recalls, "too much the terrain of sentimental pet owners. I was very explicitly told not to do it by my graduate advisers. They were very averse to anything that seemed anthropomorphic. They said it was too risky and should wait until I'd got tenure."

Times, however, have changed. "Now major textbooks, the bibles of the field, are beginning to include chapters on animal personality. Its study is pretty much accepted in various ways, either using behavioural methods - like poking squid with a brush and seeing how they respond - or getting people who know them well to rate them for traits such as curiosity."

So what had researchers discovered? Those who can't imagine what a conscientious squid or hyena would look like have found their intuitions confirmed. "There is evidence of all the Big Five in chimps, the only non-humans where we see conscientiousness - the RoboCop dimension," Gosling says. "That makes a lot of sense, because it's associated with frontal-lobe activity and inhibiting impulses, thinking before you act, planning and so on. Chimpanzees and humans are the species with the most developed frontal cortices."

But who would have expected that there are insect equivalents to the have-a-go pub brawler? Gosling reports that fellow researchers "have studied how one fruit fly is more likely to get into altercations than others, it punches and kicks others, it's less likely to back down. There are consistent differences, consistent across time and situations. You may not want to call that personality. But when people behave like that, you say that they have a personality trait of aggression."

So how has Gosling entered such strange but fascinating territory? Did he come to it from the perspective of the misty-eyed pet owner or the boy who used to hang out with the local farmer? Neither, he claims. He had "no great interest in animals" and his motives were radically unsentimental, the result of following a purely logical chain of reasoning when they were looking at the nature of personality in his first-year grad seminar at Berkeley.

"Having done half my degree in philosophy," he says, "I did a reductio ad absurdum and took a case where personality no longer makes sense. I came at it from a position that animals obviously don't have personality. That's how I remember thinking about it, wondering why animals can't have personality - and I couldn't come up with any good reasons."

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Gosling's snooping and his animal research both, no doubt, illuminate some deep and complex questions about the nature of human personality. And so does his highly entertaining book. But it also makes one wonder what he and his team of snoopers might discover about us if they descended on our houses to peek under beds and peer into closets.