Repeating letters ATCG in a colorful chart that represents a DNA sequence.

“I want to democratize science,” says biohacker extraordinaire Josiah Zayner.

This is certainly a worthy-sounding sentiment. And it is central to the ethos of biohacking, a term that’s developed a bit of sprawl. Biohacking can mean non-profit community biology labs that promote “citizen science,” or clever but not necessarily safe or innocuous garage-based experiments with computers and genetics, or efforts at biological self-optimization via techniques including cybernetic implants, drug supplements, and intermittent fasting.

Against that messy background, what should we make of Zayner? The thirty-something ex-NASA scientist, who describes himself as “a global leader in the BioHacker movement,” put his interpretation of democracy on display last October during a CRISPR-yourself performance at a San Francisco biotech conference. In that episode, he dramatically jabbed himself with a long needle, injecting his left forearm with a home-made gene-editing concoction that he said would disrupt his myostatin genes and bulk up his muscles.

Zayner sees himself, and is seen by some fellow biohackers, as a rebel hero: an intrepid scientific adventurer willing to risk his own well-being in the tradition of self-experimentation, eager...