Psychologist Robert Peck expanded Erikson's theory and identified four major psychological adjustments of middle adulthood:
- Socializing vs. Sexualizing in human relations: Redefining men and women as individuals and companions rather than viewing them primarily in terms of sex and physical attractiveness.
- Valuing wisdom vs. Valuing physical powers: Compensating for declining physical stamina by using accumulated wisdom and life experience.
- Emotional flexibility vs. Emotional impoverishment: Shifting emotional investments from one person or activity to another as children leave home or friends pass away.
- Mental flexibility vs. Mental rigidity: Remaining open to new ideas and perspectives rather than being set in one's ways.
रॉबर्ट पेक के अनुसार मध्य आयु में 'मानव संबंधों में समाजीकरण बनाम यौनीकरण' (Socializing versus sexualizing in human relations) का अर्थ विपरीत लिंग के व्यक्तियों को यौन दृष्टिकोण के स्थान पर एक सच्चे मित्र, साथी और बौद्धिक व्यक्तित्व के रूप में देखना है।