Author John Peel questions Mac's ethics in "experimenting on" Joe to further the development of the BIG RAT. On Joe as a secret agent he jokingly remarks "presumably there are no child labour laws in the future!" La Rivière's attention is drawn to Mac's line at the end of the first episode: the admonition "Don't come crying to me if you get hurt!" demonstrates the professor's willingness to "abnegate all parental responsibility". Noting "Joe 90"s subscription to "wider themes in Cold War culture", Cull likens the BIG RAT's capabilities to brainwashing but concludes that fundamentally it is "benign" technology. The stronger violence introduced in "Captain Scarlet" is sometimes evident in "Joe 90": in "Hi-Jacked", Joe kills an enemy with a grenade, while in "Project 90", Mac narrowly avoids having his head pulverised by a drill. Desmond Saunders comments: "There was an unpleasant side to series which I never really understood. There was something about it that was very strange and sinister."