The fertility deficit in many European countries is related to a low rate of second births. Understanding the factors associated with the predisposition of one-child parents to have more children could contribute to the search for solutions to this social problem. Although previous evidence highlights the role of employment and social factors, psychological factors have been poorly investigated. This study examines the relationship between different psychosocial factors (rumination, personality, life satisfaction, perfectionism, social support, parental stress, guilt linked to work–family conflict, age and child temperament) and parents’ predisposition to have more children. The sample consisted of 96 one-child Spanish parents whose child was in early childhood education (59.3% women; Mage = 37.41). The results show that one-child parents with the predisposition to have more children, compared to those without a predisposition to have more children, showed higher levels of life satisfaction, extroversion and adaptive perfectionism but lower levels of rumination and parental stress. The social implications of these findings and how they may affect parenting today are discussed.