A bumper sticker is an adhesive label or sticker with a message, intended to be attached to the bumper of an automobile and to be read by the occupants of other vehicles - although they are often stuck onto other objects. Most bumper stickers are about 30 cm by 8 cm (12 in by 3 in) and are often made of PVC.
One variety of bumper sticker is the country tag. This is typically used for cars crossing international borders, and is overseen by the United Nations as the Distinguishing Signs of Vehicles in International Traffic, being authorized by the UN's Geneva Convention on Road Traffic (1949) and Vienna Convention on Road Traffic (1968). Often the country code is displayed on the license plate itself.
People who opt to exhibit their individuality through these decals often take part in more acts of road rage. Colorado State University social psychologist, William Szlemko, found that aggressive driving is linked to the number of markers a person has on his/her car, regardless of the messages portrayed. He says this idea is linked to the idea of territoriality. The more individualized the car, the more the person tends to think of public roads as his/her own.
Due to the movement of the vehicle and changing weather conditions, the sticker needs to adhere well to the bumper surface. However, there are some "easy to remove" bumper stickers and magnetic bumper stickers. Bumper stickers can be removed with penetrating oil or a heat gun. Home remedies also include using common household items, such as WD-40 or paint thinner.
The first bumper stickers appeared shortly before World War II; they were flag-like, and attached to the bumper by wires. Gill replaced the wire attachments with pressure sensitive adhesive, and as a result bumper stickers became more widespread and practical.
Considerable variation exists around the world as to the context and purpose of stickers.
On some vehicles, some stickers are like trophy signs of WWII aeroplanes, either of locations visited or actions completed.
They have also been extensively applied to rear windows as well, where legislative measures have not banned such use. For instance in Sweden that is the normal place to put them and the bumper sticker is actually called "bakrutedekal" (rear window decal).
More recently, bumper stickers have become a route for advertising and a few companies offer to match car owners to advertisers willing to pay for the ad.
Variants of the bumper sticker have developed in recent years, including vinyl decals meant to be applied to a car's rear windshield, and chrome emblems to be affixed to the body of the car itself, generally on the rear (the "Jesus fish" and its "Darwin fish" counterpart are popular examples).