A little more info on your side as to what you are refering to would help in narrowing down what to respond to.

First, it would be of great help to narrow down the source of that claim since we cant properly talk about a general statement.

I would guess you are refering the 2009 study called "The Paradox of Declining Female Happiness" published by Bestsey Stevenson and Justin Wolfers, since thats what pops up when you type for news articles with your claim. Which claims that women had better happiness indications in the 70s(not 50s) and that a downward slope had continued up to the 2000s(mostly related to the paradox that women have to bear more workplace responsibilities in this new economy, while still being the largest contributor to unpaid labour at home).

Problem is this kind of assertation and trying to argue and explain it falls more into sociology and statistics, rather than history, so its not the best pick for this sub to answer.

Unless you are refering to another claim, which it would be great if you could specify it.