Previous literature has argued that MPs selected by party elites in a central and exclusive way are more disciplined than those belonging to parties with more inclusive and participative mechanisms of candidate selection. This hypothesis has been usually tested measuring the existence of voting blocs in parliamentary groups (taking for granted that voting blocs are the result of party discipline) and party rules on candidate selection (ignoring how selection process takes place). By using data from a survey of a representative sample of Spanish members of parliaments, we study the impact of candidate selection on how cohesion in parliamentary groups is built. Results show that where nomination for the electoral list rests on party elites, decisions are imposed through discipline. By contrast, parliamentary group cohesion is achieved through deliberative processes of decision-making in more decentralised and inclusive groups.