There are lots of reasons why people don’t always wear their glasses. They might dislike the way they look, get teased or simply feel more comfortable without them. Beyond comfort and aesthetics, though, some fear that wearing glasses too often will weaken their eyesight, and that they will increasingly rely on them more often than when first worn.

A study from Nigeria published last year found 64% of students believed that wearing glasses can damage eyes. Research in the Indian state of Karnataka put the figure at 30%, and in Pakistan 69% of people feel the same way. In Brazil, even medical staff believed that your eyes would gradually get weaker as a consequence of wearing glasses. Is there any evidence to suggest they are right?

There are, of course, two very different reasons why people wear glasses – short-sightedness, or myopia, where things in the distance are blurry; and long-sightedness, or hyperopia, where you can’t focus on things close up. Long-sightedness is often age-related: many people begin noticing in their 40-50s that it’s difficult to read in low lighting. As we age the lenses in our eyes gradually stiffen, making it harder to adjust to different distances. When people get to the stage where their arms aren’t long enough to hold a book or menu far enough away to focus on the text, they opt for reading glasses.

What’s surprising is how few trials have been conducted on the prolonged effect of wearing glasses. And from what we know there’s no persuasive evidence that wearing reading glasses affects your eyesight. Why then do so many people become convinced, anecdotally, that glasses have made their eyesight worse? People may gradually find themselves more and more dependent on their specs, but it’s because their lenses have continued to deteriorate with age. So people find themselves needing their glasses more often, leading them to conclude that the glasses must have made their sight worse, where in fact, there’s no causal relationship. Whether or not you choose to wear your reading glasses will make no difference to your eyesight in the long run (although if you have to strain your eyes to read, you might get headaches or find that your eyes feel sore).

Corrected vision

However, the situation is not the same with children. Not wearing the right glasses, or any glasses at all if they are needed, can have a long-term impact. For decades it was thought that deliberately under-correcting for short-sightedness – by giving children weaker glasses than they really needed – might slow down the elongation of the eyeball over time and thus slow down the progression of myopia. The idea was that if you wear glasses to allow you to see clearly in the distance, your eyeball tries to elongate itself when you focus on a close object in order to see it properly.

But a trial conducted in Malaysia in 2002 proved this hypothesis was so wrong it had to be halted a year early. A group of 94 children with myopia were randomised at the toss of coin either to wear the correct glasses for their prescription, or to wear glasses that left them slightly short-sighted. When the study began the children were between the ages of nine and 14, and for the next two years the length of their eyeballs were measured at regular intervals. Contrary to an earlier, smaller study from the 1960s, the children who wore the weaker glasses showed a greater elongation of the eyeball over time. In other words their eyesight was gradually getting worse.