Why do you need to turn off all your electronic devices before an airplane takes off and lands?

We’ve been taught to fear the interference of our portable devices and an airplane’s sensitive electronic systems. And despite being told to turn off our darn phones, four out of ten passengers, it seems, disregard these warnings, since there’s little hard evidence behind claims that electromagnetic emissions from devices can muddle airplane computers. Still, there’s ample anecdotal corroboration, like the flight where a 30-degree navigation error was rectified simply by asking a passenger to turn off a portable DVD player. The issue is still being debated, so if in doubt, flick that Off switch. We might, eventually, long for the days when a chatty seatmate couldn’t make an hour-long phone call.