The Organizational-Activational Hypothesis

Critical Periods in Development

Permanent vs Temporary Hormonal Effects

The Prenatal Testosterone Surge in Male Development

Congenital Adrenal Hyperplasia (box)

Androgen Insensitivity Syndrome (box)

Pubertal Changes in Brain Structure

Menstrual Cycle Effects on Brain Function

Pregnancy Effects on Brain Function

Contraceptive Effects on Brain Function

Menopause Effects on Brain Function

The organizational-activational concept is a foundational framework in behavioral neuroendocrinology that explains how hormones influence the development of sex-typical behaviors and characteristics through two distinct phases.

Organizational Effects

The organizational phase occurs during critical developmental periods, primarily during prenatal development and early postnatal life. During this time, hormones (particularly androgens like testosterone) permanently “organize” or structure the brain and other tissues in sex-typical ways. These effects:

• Are irreversible once the critical period closes
• Create lasting structural differences in brain regions, neural circuits, and peripheral tissues
• Establish the biological foundation for later sex-typical behaviors and responses
• Occur through mechanisms like cell proliferation, migration, differentiation, and programmed cell death

For example, prenatal testosterone exposure masculinizes certain brain regions and creates the neural substrate that will later support male-typical behaviors when activated by hormones.

Activational Effects

The activational phase occurs throughout life when circulating hormones “activate” the previously organized neural circuits and biological systems. These effects:

• Are typically reversible and depend on current hormone levels
• Trigger the expression of behaviors and responses for which the organism was organized during development
• Can be turned on and off as hormone levels fluctuate
• Include effects on sexual behavior, aggression, cognitive patterns, and other sex-typed behaviors

Application to Sex and Gender Development

This framework helps explain several aspects of sex and gender development:

Typical Development: Prenatal testosterone organizes male-typical brain patterns, which are later activated by pubertal and adult testosterone to produce masculine behaviors and characteristics.