People’s social environment is key in explaining their behaviour and preferences, including how many children people want and have. Unfortunately, obtaining information on the social environment, for instance through collecting personal network data, is difficult and often burdensome to participants. In this talk, I will first report on my experiences with collecting large personal network data (25 alters) from a representative sample of Dutch women. I’ll discuss the trade-offs researchers face when designing network studies between the burden to respondents and the reliability of characteristics of networks. Data quality is further discussed, including the usefulness of the concept “friend”. In the second part of the talk, I use these data to test evolutionary and sociological ideas about the breakdown of kin networks in contemporary populations as an explanation for the fertility decline. I’ll further discuss how personal network data can inform on social influences on fertility behaviour.