Eugenics is a great idea!
Explanation
The Joke
Two people are discussing eugenics. One says, "I don't see what's so bad about eugenics. Oh sure, there's some bad history, but we could make people smarter, stronger, and more empathic." The other enthusiastically agrees: "Great idea! Tell you what: I have a machine. Since eugenics is so great, let's make it happen in the past." She then asks which scientifically advanced previous generation the first person would like to have had complete control over their physical and mental attributes -- 1960s America? 1930s Britain? 18th century France? The first person objects: "That's not fair! Those people had prejudices! Even the one whose ethics was the best, France, was that it worked out in summer 2013." The other responds: "What about all the people using it think you're wrong?" The first says: "I believe we can sterilize them in the water supply."
The Humor
The comic brilliantly exposes the fundamental flaw in pro-eugenics arguments by using a simple time-travel thought experiment. The person advocating for eugenics imagines themselves as the one making the decisions with today's enlightened values. But when asked to consider past generations having the same power, the advocate immediately recognizes that those people's prejudices and limited understanding would have led to horrific outcomes. The deeper joke is that the advocate fails to realize the implication: future generations will almost certainly view our current values and knowledge as similarly flawed and prejudiced. The final punchline -- casually suggesting sterilizing dissenters through the water supply -- reveals the authoritarian impulse lurking beneath the "reasonable" eugenics argument, showing how quickly even well-intentioned eugenics thinking slides into monstrous territory.
References
The comic references the real historical abuses of eugenics programs, including forced sterilization programs in the United States (particularly active in the early-to-mid 20th century), Nazi Germany's racial hygiene programs, and various European eugenics movements. These programs disproportionately targeted minorities, the disabled, and the poor.