An interesting thought, but I think this is one of those social science hypotheses that you're just not gonna be able to prove.
Your experimental measures are not strong enough to answer this question. And if we find some way of measuring society-wide guilt other than your subjective selection and interpretation of lines from ancient plays, and if we suppose hypothetically that this still backs up the conclusions of your incredibly subjective survey, there can very easily be a common cause (technological progress, increasing "liberal drift"), reverse causation (maybe cultural success allows for more guilt), or just random chance at work since the sample size is small.
Related: The Psychological Diversity of Mankind, An African Folktale, many of the more interesting infanticide & abortion debates on this site
A fascinating post that however might need some background reading, most relevant material is linked in the article itself. I encourage reading up on the material.
Link to article.
Stories about changelings replacing babies and the recommended course of action being basically to expose the child is not a human universal, they are found only in European cultures. These rely more heavily on guilt and less on shame to regulate behavior than most other human societies. This may not be a coincidence. The stories look like they work as a ready made rationalization to reduce guilt from infanticide. Common problems often acquire common solutions like this.