Why is an old iPhone still worth hundreds of dollars? New research suggests a surprising explanation: hoarding.

Why are people hoarding $9 billion in old iPhones?

More than half of American consumers say they have two or more unused cell phones in their household, according to a new survey released Friday. The trade-in value of all those old gadgets: $34 billion, according to resale site SellCell.com — which has a vested interest in tapping into this possible gold mine. Old iPhones account for roughly a quarter of that sum. Apple AAPL, -2.95% has been able to release a new model every year since 2007 — 85 million phones and $50 billion in revenue — partly because it’s easy and inexpensive for customer to sell their old ones for new models. But clearly large numbers of users never part with their old devices.