Tex. Fam. Code §153.131
Presumption That Parent To Be Appointed Managing Conservator

Subject to the prohibition in Section 153.004 , unless the court finds that appointment of the parent or parents would not be in the best interest of the child because the appointment would significantly impair the child's physical health or emotional development, a parent shall be appointed sole managing conservator or both parents shall be appointed as joint managing conservators of the child.

It is a rebuttable presumption that the appointment of the parents of a child as joint managing conservators is in the best interest of the child. A finding of a history of family violence involving the parents of a child removes the presumption under this subsection.