Explores the relationship between incarceration & family structure among new parents. Data drawn from the Fragile Families study of young families included 656 married & unmarried families from Oakland & Austin. New mothers were interviewed shortly after giving birth & follow-up interviews with both parents were conducted when the child was 1, 2.5, & 4 years old. The results of regression analysis indicate that incarceration narrowly influences whether a father is present in the household & does not affect the likelihood of marriage among cohabitating couples. Incarceration has a destabilizing effect on young, low-income families by absence of the father while he serves time & reducing his employment/earnings prospects after release. Growth in the prison population, especially among young people of color, contributes to the increase in both family instability & single-parent families among minority groups. It is concluded that the formal social control of prison undermines the informal social control of families & contributes to crime in high incarceration area. The policy implications are discussed. 5 Tables, 20 References.