Mignon Eberhart, "American's Agatha Christie," was one of the major figures involved in the middle twentieth century in shifting the focus in crime fiction from strict detection to atmospherics and emotions. In a 1944 interview she defended herself against charges from members of the orthodox school of mystery puzzling, who deemed love interest in crime fiction an unpardonable distraction from detection:
Mrs. Eberhart disagrees with those critics who say romance has no place in mystery. "A mystery is about people. Your characters are like a still pool until the murder happens. The murder is a stone hurled into the pool, disturbing its smooth surface and revealing its depths. Out of ten or twelve characters thus disturbed, isn't it reasonable to suppose that some two of them will have a romantic attraction for each other? The exclusion of romance in a mystery seems to me more artificial than letting it takes\ its course."
Romance or "love interest" was a much debated element in mystery fiction in the 1920s and 1930s. While traditionalists proscribed it as anything more than a minor feature of a mystery, confined to bland secondary characters, in the 1930s it became something that was seen more and more in mysteries. The British Crime Queens Dorothy L. Sayers, Margery Allingham and Ngaio Marsh all had their detectives fall in love and marry over the course of their series and in Mignon Eberhart's books the murders that take place are most significant as obstacles to the happy resolution of the heroine's love life.
How much love interests you in a mystery?