One of the great failures of classical Marxism has been the lack of development of class solidarity. Marx expected the working class to become more conscious of its common interests and thus develop into a powerful political force.But this never happened to the extent that Marxists hoped.

Why not? One possible reason lies in reference group theory. This says that individuals compare themselves to their own social circle rather than to everyone, with the result that we envy colleagues and former classmates who have done slightly better than us rather than the super-rich. This erodes class solidarity.

Some laboratory experiments (pdf) by Philip Grossman and Mana Komai have shown how common such in-group envy is, and how damaging it can be.

They split subjects randomly into two groups - rich and poor, with the rich given a higher endowment and the chance to invest at higher returns than the poor. After a while of investing, and increasing class inequality, subjects were then given the opportunity to spend some money in order to destroy part of another's wealth.

If people were free of envy, they'd never take up this offer as it always impoverished them. But in fact, such choices were quite common. In total, subjects chose to destroy part of another's wealth in almost a quarter of the instances in which they could; 2346 times out of 9600 choices.

However, attacks by the poor on the rich were only a minority of all attacks - 619 of the 2346. Almost as often (509 times), the poor class attacked their fellow poor, with most of those attacks being upon people poorer than themselves. And most of the attacks upon the rich came from other rich folk.

This tells us that there is indeed some inequality aversion; people will pay money to reduce inequality. But this is only part of the story. Folk are also concerned with their individual relative status. So they'll pay money to hold down people who are slightly worse off than themselves, and to bring those slightly above them down a peg or two.

Of course, there are countless real world analogies to this behaviour. Old money sneering at new money, the rich complaining about the super-rich, "white trash" being racist and "strivers" attacking "shirkers" are all examples of within-class conflict. What's striking about this experiment is that such behaviour emerges so easily, without the aid of ideology or media manipulation.This suggests that the lack of development of class solidarity has some deeper-rooted causes than ideology alone.

For a Marxist, this is depressing stuff. But it should also concern any liberal or democrat.It suggests that people might support policies that hurt other poor people - for example, welfare cuts or immigration controls - even if they themselves are harmed by such policies. In this sense, people's preferences aren't necessarily the same as their narrow material interests.