Scientists Replicate Infamous 1960s Planarian Memory Experiments With Surprising Results

A team of researchers at Harvard University attempted to replicate the infamous 1960s planarian worm learning and memory transfer experiments, but reported in April 2026 that modern flatworms failed to learn. In the mid-20th century, psychologist James McConnell claimed that planarian flatworms could be classically conditioned and that memories could be transferred to naive worms via cannibalism. While modern researchers could not replicate learning in planarians, memory transfer research in other organisms, such as sea slugs and roundworms, has gained renewed scientific interest.

The Quest to Replicate McConnell's Worms

Led by neuroscientist Sam Gershman, the Harvard team set out to follow the rigorous protocols established by McConnell's former student Alan Jacobson. Research assistants and postdoctoral fellows, including Zachary Kelso and Maddie Snyder, traveled across the United States—from the Charles River in Boston to streams in Oregon and lakes in Michigan—to source wild planarians. Despite trying various strains and following historical instructions precisely, the modern flatworms could not be conditioned to associate light with a shock.

To ensure accuracy, the team even tracked down surviving McConnell collaborators, Daniel and Reeva Reeva Kimble, who preserved the print archive of *The Worm Runner's Digest*. Despite reviewing these historical methods, today's planarians remained entirely uneducable.

Explaining the Discrepancy

Scientists have proposed several hypotheses for why 1960s planarians appeared to learn while modern ones do not:

• Scoring Bias: Researchers in the 1960s may have misinterpreted normal worm movements for the specific light reaction, influenced by the charismatic leadership and expectations of the era.
• Environmental or Biological Shifts: Changes over six decades, though considered less likely by some researchers, could theoretically affect modern flatworms.
• Evolutionary Context: Planarians possess extraordinary regenerative capabilities, raising questions about why such resilient organisms would rely on traditional associative memory.

Modern Memory Transfer in Other Organisms

While planarian learning appears to be a dead end, studies involving other species have yielded more promising results:

• Sea Slugs: In 2018, researchers at UCLA demonstrated that sensitization could be transferred between *Aplysia californica* sea slugs via direct injections of genetic material.
• Roundworms: Research at Princeton University and the Indian Institute of Science indicated that *Caenorhabditis elegans* can learn to avoid hazards and potentially share behavioral information through genetic particles or extracellular vesicles.

Gershman's lab is now shifting its focus from planarians to *C. elegans* to study memory at a molecular level. Readers interested in verifying these claims can review the preprints published on bioRxiv or consult historical accounts in scientific literature.