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Based upon studies conducted over the last two decades that show head injuries can be reduced if children wear bicycle helmets, some jurisdictions have passed laws requiring that children under the age of 12 wear helmets when they ride their bicycles. A surprising result has been observed, however. Even though a large number of children do wear bicycle helmets, more head injuries than expected are continuing to occur. And this is the result even though the studies have shown that wearing a helmet reduces the incidence of head injuries.
Which one of the following statements, if true, might suitably explain the unexpected finding?