Professor Cary Cooper of the Alliance Manchester Business School says in the main, milk going missing is down to people rationalising it as being communal.
"They don't see it as a crime. They see it as sharing," he says.
Taking people's lunch is different. "That's a bit nasty and probably a revenge attack", he says.
The thief could think: "I'm going to pay her back, take her lunch from her, and then say: 'I wonder who did that? That happened to me too. I wonder what the world is coming to?'"
He said people taking food for the thrill of it would be "very unusual".
"There's more exciting things to do," he says.
People get annoyed about food and milk being taken because they see it as a betrayal of trust, he adds.