Testosterone (Testosterone - The Hormone that Dominates and Divides Us)

This definitive text is the complete Japanese translation of Dr. Carole Hooven's acclaimed book, T: The Story of Testosterone, the Hormone that Dominates and Divides Us. It offers a clear, scientifically grounded, and rigorously objective account of how testosterone—a single, pivotal hormone—plays a foundational role in shaping both biological sex differences and the intensely complex issues surrounding gender.
 
Testosterone is classified as an androgen, a group of hormones often called the "male hormones." This class includes other crucial players such as dehydroepiandrosterone (DHEA) and dihydrotestosterone (DHT). Androgens act by binding to specialized androgen receptors (ARs) in cells across the human body.
 
Once bound, these hormones trigger two major, distinct types of effects. The first is the masculinizing effect, responsible for developing secondary sexual characteristics like body hair and external genitalia, and regulating libido. The second is the vital anabolic effect, which promotes significant muscle growth, raises red blood cell counts, and accelerates bone metabolism—all essential processes that underpin robust human growth and development.
 
However, the book reveals a critical subtlety: stronger androgenic effects do not simply result from higher hormone levels. The crucial factor that creates the vast differences in testosterone’s impact—shaping personality and diversity—is not merely the secreted amount of hormone, but rather the highly complex way target cells receive and process its signals.
 
The core of this individual variation lies in genetic diversity. A key element explored is the genetic variation in the androgen receptor (AR) gene itself. If a mutation or variation occurs, the hormonal signal may not be properly transmitted or received by the cell, consequently reducing the hormone's biological effects. These variations exist on a broad spectrum, from those that completely disable the receptor to others that allow for varying degrees of partial activity.
 
The book further details another significant element: the CAG repeat sequence within the AR gene. The specific number of these repeats varies uniquely among individuals, fundamentally altering the body’s intrinsic sensitivity and responsiveness to androgens. A general trend shows that a shorter number of repeats correlates with higher sensitivity, while a greater number correlates with lower sensitivity. Furthermore, minute changes called single nucleotide polymorphisms (SNPs)—variations at a single base position—act as fine-tuners, subtly modulating the overall receptor function.
 
Thus, the action of androgens is governed by a sophisticated interplay between hormone levels and this extensive genetic diversity in receptor function. This inherent biological variation powerfully explains the wide range of testosterone’s effects seen among humans, fundamentally challenging the rigid, long-held idea that “sex” is a simple, unambiguous male-or-female binary. Building upon this rigorous scientific foundation, Dr. Hooven draws insights from fields like evolutionary biology and embryology to explore the far-reaching influence of testosterone—how it profoundly shapes human life and even influences the social structures and norms we live by.
 

(Written by TSUBOI Takashi, Professor, Graduate School of Arts and Sciences / 2025)