Mo. Rev. Stat. § 375.296: Additional damages for vexatious refusal to pay.

In any action, suit or other proceeding instituted against any insurance company, association or other insurer upon any contract of insurance issued or delivered in this state to a resident of this state, or to a corporation incorporated in or authorized to do business in this state, if the insurer has failed or refused for a period of thirty days after due demand therefor prior to the institution of the action, suit or proceeding, to make payment under and in accordance with the terms and provisions of the contract of insurance, and it shall appear from the evidence that the refusal was vexatious and without reasonable cause, the court or jury may, in addition to the amount due under the provisions of the contract of insurance and interest thereon, allow the plaintiff damages for vexatious refusal to pay and attorney's fees as provided in section 375.420. Failure of an insurer to appear and defend any action, suit or other proceeding shall be deemed prima facie evidence that its failure to make payment was vexatious without reasonable cause.