Girls attain reproductive maturity about four years after the first physical changes of puberty appear. High-quality romantic relationships are associated with higher commitment in early adulthood  and are positively associated with self-esteem, self-confidence, and social competence. Some boys may develop gynecomastia due to an imbalance of sex hormones , tissue responsiveness or obesity. Everyone has a self-concept, whereas Erik Erikson argued that not everyone fully achieves identity. While children that grow up in nice suburban communities are not exposed to bad environments they are more likely to participate in activities that can benefit their identity and contribute to a more successful identity development. Peer groups can have positive influences on an individual, such as on academic motivation and performance. Despite the common [ according to whom? Toronto, Ontario: Pearson Education Canada.