10月5日 私たちの体のDNAが心を作っている話です。

The Problem with "Educational Disparity"

Takeda Tetsuya opens this episode with behavioral geneticist Robert Plomin's book, published in Japan as *Kokoro wa Iden suru* (河出書房新社) and originally titled *How DNA Makes Us Who We Are*. What drew Takeda to the book was a nagging unease with the phrase "educational disparity" (教育格差)—the idea that a child's academic performance is determined by the socioeconomic conditions of their parents: income, educational background, occupation.

The concept is routinely used to argue that closing the income gap will close the achievement gap. Takeda pushes back bluntly: "If you made everyone's income exactly equal, would everyone score the same on a test? Of course not." Give any group a test and scores will spread from zero to a hundred. Equalizing household income does not flatten that spread. Takeda dismisses "educational disparity" as a term that works only in politics—an inaccurate concept that ignores genetic factors—and says Plomin's research gave scientific grounding to that intuition.

Dr. Plomin's Twin Studies—Body Weight and Genetics

Plomin's method for separating the contributions of nature and nurture is long-term longitudinal tracking of twins—both identical and fraternal—over several decades. The results for body weight alone are striking. We tend to assume weight is a product of diet and lifestyle—"nurture"—but the data show that **70% of body weight is influenced by genetics**. Twins separated shortly after birth and raised in different households grow to resemble their biological parents' body types more closely with each passing year.

Takeda illustrates this from his own experience: his weight had climbed to 76 kg before dropping to 63 kg after age 60, leaving him with a physique that mirrors his late father's. His female assistant added that she has noticed herself taking on her mother's body shape. The pattern, Takeda notes, runs true across the data.

Television Habits Are Partly Inherited Too

Plomin also examined television-viewing behavior—specifically, whether a person switches the set on the moment they get home and leaves it running, or turns it off as soon as they're done with it. Even this seemingly trivial habit carries a genetic signature. When adopted twins raised in separate households were compared, their daily viewing time, their tendency to leave the TV on, and their preferred genres—sports, drama, comedy—all matched at rates higher than chance. **One-third of total TV viewing time and 25% of genre preferences can be explained by genetics.** "You wouldn't think of that as genetic," Takeda observes, "but it's hard to see because each of us only has one life to compare"—though he notes that siblings in large families often share strikingly similar viewing patterns.

The Heritability of Academic Performance—and the Critique of "Educational Disparity"

Plomin applies a specific metric—heritability—to academic performance as well. At the elementary-school level, the heritability of academic achievement stands at **25%**. Plomin then tracked the same cohorts through middle school, high school, and university, showing that this figure shifts as children advance through the education system.

Takeda draws the conclusion plainly: "DNA is the single most important systematic factor in determining what kind of person we become. Through study after study, Dr. Plomin shows just how inaccurate political terms like 'educational disparity' really are."

Genetics as Tendency, Not Destiny—Takeda's Story of Self-Discovery

Takeda is careful to frame genetics not as determinism but as **tendency**. DNA lays a probabilistic blueprint; it awakens gradually through the choices a person makes in their environment—which subjects to love, which friends to seek out, which habits to cultivate. Those choices are themselves nudged by genetics, but they remain choices.

He anchors the idea in two personal memories. As a second-grader who ranked in the low twenties out of thirty classmates academically, he chose a morning-glory (asagao) observation diary as his summer free-study project. Day after day he recorded the plant's journey—from seed to flower to new seed—in colored-crayon illustrations. His female homeroom teacher singled out the diary at a class meeting and awarded it a silver "effort" sticker. "At age 8 I felt for the first time in my life what it is to have blood surge with a sense of achievement," Takeda says.

By sixth grade he had discovered a collection of biographies at the school library—roughly 200 great figures from world history—and became so absorbed that he memorized the life story of every person in the book. Looking back, Takeda reads both experiences as DNA whispering his true nature to him: a persistent capacity to observe things closely and to tell their stories. That, he says, is the tendency his genes encoded.