Massachusetts is a strict liability state.
Massachusetts has one of the best laws for the protection of dog bite victims, especially young children. A dog owner or keeper is strictly liable for dog bites unless the victim was trespassing, teasing, tormenting or abusing the dog, or was committing another tort. A child under the age of 7 is presumed to have done none of those things, but the presumption is rebuttable.
General Laws of Massachusetts, Chapter 140: Section 155. Liability for damage caused by dog; minors; presumption and burden of proof.
Section 155. If any dog shall do any damage to either the body or property of any person, the owner or keeper, or if the owner or keeper be a minor, the parent or guardian of such minor, shall be liable for such damage, unless such damage shall have been occasioned to the body or property of a person who, at the time such damage was sustained, was committing a trespass or other tort, or was teasing, tormenting or abusing such dog. If a minor, on whose behalf an action under this section is brought, is under seven years of age at the time the damage was done, it shall be presumed that such minor was not committing a trespass or other tort, or teasing, tormenting or abusing such dog, and the burden of proof thereof shall be upon the defendant in such action.
A person who is injured by a dog in this state also may use the other common grounds for liability. They are scienter, negligence, and negligence per se. For further information about these causes of action, see Legal Rights of Dog Bite Victims in the USA.