If told that they own it, people are less likely to trade a possession to obtain something owned by someone else. This is the case even when there is no cause for attachment, or even if the item was only obtained mere moments ago.
A classic experiment was done by Kahneman, Knetsch & Thaler (1990) on the Endowment Effect. People were each given a coffee mug and then given the choice to sell or swap it for an equally-priced alternative which, in this case, was a pen. Fascinatingly, they found that people wanted to be paid twice the money for their coffee mug as they’d themselves be willing to pay for the pen.
Elsewhere, workers have been found to work harder to maintain a pre-awarded pay packet bonus than a separate potential bonus they don’t yet feel a sense of ownership over (Hossain and List, 2009). Also, contrary to thought regarding the Endowment Effect as a mistake diminishing as we mature, children (even unsophisticated ones) have been found to be susceptible (Harbaugh et al, 2001), as well as Great Apes, would you believe (Flemming et al, 2011)!?
Why is this? Well, a lot is down to our natural tendency as humans to be loss-averse - basically, the fact that once you own an item, forgoing it feels like a terrible loss. The best example right now is your mobile phone - we protect and personalise it with skins, covers and apps, and are highly unlikely to exchange it for an equivalent item. Do you remember the last time you lost your phone?
However, a curious bit of recent research carried out in northern Tanzania suggests that not everyone has this bias! Indeed, it may be an irrational habit of the minds of people living in market-orientated societies. Two groups from the Hadza tribe - one connected and trading with tourists, and another more remote and without trade - were both given biscuits. Each were then shown a lighter, and asked if they’d like to trade. The remote Hadza had a 50-50 chance of trading - i.e. they were rational, not preferring one product over the other. This is in stark contrast to the market-orientated Hadza, who had a 75% chance of keeping their biscuits. Remote Hadza therefore had no endowment effect.
It’s therefore important not to make too many generalisations about the human mind in general; we certainly need to factor in our WEIRD (Western, Educated, Industrial, Rich and Democratic) perspective on things. Nevertheless, the Endowment Effect is a very strong and highly consistent bias that affects the vast majority of people you interact with and sell to every day.