Based on findings showing that birth order and the presence or absence of siblings influence personality (Adler, 1927; Stewart & Stewart, 1995; Sulloway, 1996), especially when that sibling has a disability (Meyer, 1997; Nesa, 2005), this research aims to investigate the effect of such variables on adolescents' moral reasoning. A sample of male and female adolescents from three secondary schools in Dublin, aged 12-18 (n = 164), were given the Prosocial Reasoning Objective Measure (PROM) questionnaire (Cario, Einsberg & Knight, 1992). Only-children scored significantly higher in needs-orientated reasoning type then sibling adolescents while sibling adolescents scored significantly higher in internalised type then only-children. Males scored significantly higher in approval-oriented type then females while females scored significantly higher than males in needs-oriented type (p < 0.05). However no significant difference was found between those who had a sibling with a disability, the different age groups or between the different family rank orders. Cultural background was not taken into account while further research may consider number of siblings and severity of disability of a sibling.