Clinical Journal of Sports Medicine has published a study that indicates that sports carries risk of long term brain injury.

One of the reasons the study was conducted is the fact that some earlier studies revealed memory and attention problems in college soccer players.

Researchers in the study used high-resolution MRI brain scans to compare brains of 10 soccer players with 10 other men who have never played the sport. The MRI results showed reduced gray matter in the brains of soccer players.

Grey matter is the brain tissue that controls thinking and memory. Gray matter reduction was seen in a part of the brain called the anterior temporal cortex — which is consistent with effects from repeated knocks to the front of the head.
There is a big risk of concussion and brain injury in a game such as soccer. Heading the ball or players colliding with one another on the field are situations that could lead to these unfortunate incidents.

From the 10 players in the study, only two suffered a mild concussion in the past. This is why researchers could not relate heading to reduced grey matter and until further studies are conducted, soccer should not be considered as an entirely benign sport.