eye, excessive blinking

I'm sometimes asked about a child who constantly (it seems to the parents) blinks his or her eyes. Reasons for this include:

Allergic irritation of the eyes. This is easily discernible on physical examination and is treatable with cromolyn sodium or other non-steroidal eye drops.
An annoying habit. It may start with reason one. Parental attention and comment reinforces the constant blinking, just as with thumb sucking, nail biting, etc. Message delivered. Message received?