Brain Wiring: Corpus Callosum and Cognition

Scissors, spiral notebooks, cultural taboos — lefties have it rough. Jessica Wynn explores why 10% of us are southpaws here on Skeptical Sunday.

Welcome to Skeptical Sunday, a special edition of The Jordan Harbinger Show where Jordan and a guest break down a topic that you may have never thought about, open things up, and debunk common misconceptions. This time around, we’re joined by writer and researcher Jessica Wynn!

Full show notes and resources can be found here: jordanharbinger.com/1272

On This Week's Skeptical Sunday:

• Left-handedness isn't controlled by a single gene — it's a cocktail of genetics, prenatal environment, and random brain development. The rare gene variant TUBB4B shows up three times more often in lefties, but even identical twins sharing 100% of their DNA can have different dominant hands.
• Your handedness was essentially decided before you were born. Ultrasound studies reveal fetuses showing consistent hand preferences by 18 weeks — those who suck their left thumb in the womb typically become left-handed. Your fetal position and which direction you turned your head may have sealed your southpaw fate.
• Left-handers possess a larger corpus callosum — the neural bridge connecting both brain hemispheres — enabling faster communication between the two sides. This biological quirk gives lefties advantages in multitasking, creative problem-solving, and activities requiring rapid information processing like video game
• Cultural conditioning has historically punished left-handedness, associating it with everything from uncleanliness to witchcraft. Schools once forced children to switch hands, and in some cultures today, using your left hand for eating remains taboo — a lingering echo of pre-toilet paper hygiene practices.
• Celebrate your brain's beautiful asymmetry — whether you're a lefty or righty. Try using your non-dominant hand for everyday tasks on August 13th (International Left-Handers Day) to appreciate how deeply ingrained handedness really is. It's a humbling reminder of how wonderfully weird human development can be.
• Connect with Jordan on Twitter, Instagram, and YouTube. If you have something you'd like us to tackle here on Skeptical Sunday, drop Jordan a line at jordan@jordanharbinger.com and let him know!
• Connect with Jessica Wynn at Instagram and Threads, and subscribe to her newsletters: Between the Lines and Where the Shadows Linger!

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