How we define family and form relationships is, in some large measure, determined by the relationships we had in our family of commitment origin social intimacy heritage will give

The word that bests fill in the blank is ORIGIN. 

Family of origin refers to the first social group a person belongs to, which is typically the person's biological family or in some cases, an adoptive family. Truly, the way we relate with people in our family of origin determines, in some large measure, how we define family and form relationships as we go through life.