Although it has been claimed since 1962 that Marx was the father of Helene Demuth's illegitimate son, according to Terrell Carver, "this is not well founded on the documentary materials available".
Meskipun sejak 1962 diklaim bahwa Marx adalah ayah dari putra kandung Helene Demuth, menurut Terrell Carver, " ini tak berdasarkan pada material-material dokumenter yang tersedia".