The article investigates the different types of social mechanisms responsible for the interdependence of couples' reproductive preferences predicted by diffusion models of fertility and family behavior. First we analyze the transcripts of in-depth interviews carried out with 54 women in the northern part of Italy. The rich information on observations and conversations about fertility and family choices with relatives and peers enables us to dis- tinguish four different ways in which social interaction influences reproductive preferences, namely social learning, social pressure, subjective obligation and contagion. Second, we show how the efficacy of each mechanism affecting fertility behavior depends on the kind and the structure of personal relationships involved in the interaction. Finally, we discuss the ways in which individual attitudes and values associated with the transition to parenthood are produced and negotiated in face-to-face interactions, and the importance of focusing on the process of preference-formation and modification for understanding fertility behavior.