Background: Research indicates that both full- and half-siblingships develop enduring social relationships, if the siblings have the opportunity to interact during childhood and adolescence.

Objective: To estimate: (1) how much time half- and full-siblings are exposed to each other during childhood and adolescence; (2) how half-sibling exposure is conditional on birth spacing and residency; and (3) how parents' social vulnerability is associated with different levels of lifetime exposure to half-siblings.

Methods: Swedish register data is used to calculate exposure to half-siblings based on birth spacing and registered residency for all full- and half-siblings in the 1994 birth cohort.

Results: A substantive share of half-siblings are less exposed to each other due to lengthy birth spacing and residency patterns. By age 18, 26% of the birth cohort have had a half-sibling who is also no older than 18 for at least one year; 13% of the birth cohort have had a half-sibling who is no older than 18 for up to 10 years; 8% of the birth cohort have been registered in the same dwelling as another half-sibling for eight years or more. Parents’ social vulnerability does not predict exposure to halfsiblings among the population that has at least one half-sibling by age 18.

Conclusions: Even though half-siblings constitute a large share of all siblings, full-siblings will likely make up the vast majority of the siblingship-like relationships because so many halfsiblings are unable to interact during childhood or adolescence due to extensive age differences and/or because they do not coreside.

Contribution: This study quantifies the role of birth spacing and residency patterns for the exposure to full and half siblings across childhood. It highlights the benefits of including a population perspective for understanding full and half sibling social relationships.