William Jankowiak and Edward Fischer (1992) examined 166 tribal cultures, and found that passionate ...

William Jankowiak and Edward Fischer (1992) examined 166 tribal cultures, and found that passionate love existed in at least 147 of them. What is thought to be the main reason that a lack of passionate love was found in the other 19 tribal cultures?
 
  a. those tribes were more fearful of outsiders, than the other tribes
  b. there was ethnographic oversight of the questions used in the surveys
  c. there is a higher degree of matriarchal dominance than in the other tribes
  d. there is a higher degree of patriarchal dominance than in the other tribes