A recent study of inexpensive earrings conducted by a pair of scientists at UC-SF found that almost a third of the under-$50 pairs they tested contained nickel. Why does this matter? In my years as a jeweler I became acutely aware of just how many people are allergic to the metal. Nickel dermatitis results in very sore, red and swollen ears and eczema-like symptoms.
The study found that a higher percentage of earrings on American shelves targeted to younger wearers contained nickel. They didn't find an correlation between the country of origin and the propensity for nickel.
Fortunately, there is a simple fix for many cheap earrings. If they are dangles, replacing ear wires made with nickel with those made with surgical steel should fix the problem. Fixed posts can also usually be swapped for stainless steel ones, although the cost of having your jeweler do so may exceed the value of the earrings.
Gold and platinum jewelry doesn't contain nickel.