Background : Support from families can reduce costs of reproduction and may therefore be associated with higher fertility for men and women. Family supportiveness, however, varies both between families - some families are more supportive than others - and within families over time - as the needs of recipients and the abilities of support givers change. Distinguishing the effects of time-invariant between-family supportiveness and time-varying within-family supportiveness on fertility can help contribute to an understanding of how family support influences fertility. Objective : We distinguish 'between' and 'within' families for several types of support shared between parents and adult children and test whether between- and within-family variation in support associates with birth timings. Methods : We use seven years of annually collected LISS panel data from the Netherlands on 2,288 reproductive-aged men and women to investigate the timing of first and subsequent births. Results : We find between-family support is more often associated with fertility than is within-family support, particularly for first births and for women. Emotional support is generally associated with earlier first births for both men and women, while results for financial and reciprocal emotional support are mixed. There is some indication that the latter kind of support positively predicts births for men and negatively for women. Conclusions : Our results suggest that feeling supported may be more important than actual support in reproductive decision-making in this high-income setting. Contribution : We apply a method novel to human demography to address both a conceptual and methodological issue in studies of families and fertility.