The study sets out to investigate the relationship between low fertility and new patterns in the family and non-family sectors. It examines the social implications of childlessness, single-child families and other family sizes with an emphasis on questions of social cohesion.Firstly a theorical perspective on childlessness is given. This is followed by an analysis of the impact of changes in birth order-specific fertility on family size using the results from a simulation study which analyses how family sizes change when the level and timing of age - and order-specific fertility change. The final section discusses possible consequences for social cohesion and social exclusion of the trends identified in the previous sections with a focus on poverty.