There were actually four different modifications. The Harrington Clan descended from colonists who had undergone the Meyerdahl Beta modifications, as did about twenty-five percent of the human population of Sphinx. (HH9)
The Beta version of the Meyerdahl modifications included what was known as an "IQ enhancer", although it seemed to lack the aggressive side effects that plagued the "Super-soldier" modifications on Old Earth. Most of the Harrington descendants of Meyerdahl Beta recipients placed in the 99th percentile in intelligence tests, with only three known descendants rating lower than 95th percentile.