In a society with a strong two-child family norm, but where children tend to reproduce the fertility behavior of their parents, it is relevant to study the childbearing patterns of only children. Do they follow the stream or do they follow their parents? There is very little previous research on this topic, but there is some evidence that show that only children are not more likely to prefer a one-child family themselves. Using GGS data from Sweden and Norway, this study aims to understand if being an only child is a determinant for having an ideal family size of one child, or having a completed family size of one child. The study is limited to those who want at least one child and to those who have at least one child (i.e. the study excludes childless). The analysis is conducted with binomial logistic and linear (OLS) regression methods. The study finds that only children more often do prefer a family size of one child and more often end up with a final family size of one child, compared to sib children. However, the association between being an only child and having a one child preference is also influenced by socio-economic status in the family of origin. If the parents’ education is high (post- secondary/tertiary), the likelihood of someone having a one-child preference decreases. The relationship between being an only child and having a final family size of one child is partly mediated by the experience of parental separation and/or own separation, where both factors increase the odds of stopping at one child. The conclusion and main contribution of this study is that there seems to be a family of origin socialization mechanism influencing the fertility of only children, making them more likely to both prefer and have a one-child family, compared to sib children. Thus, only children are more likely to deviate from the two-child norm, compared a person with siblings.