Working moms laugh and smile more than SAHMs

In her guest column, titled The Triumph of the Working Mother, Stephanie Coontz lays out what she sees as clear evidence that working mothers are less depressed and angry, more inclined to smile and laugh, or at least remember smiling and laughing, than those moms who stay at home with their children.

"In a 2012 Gallup poll, stay-at-home mothers in low-income families were less likely than employed moms at the same income level to report that they had smiled, laughed, or enjoyed themselves “yesterday.”

Coontz bolsters this theory of increased well being for working moms with plenty of definitive-sounding statements, including:

"At all income levels, stay-at-home mothers report more sadness, anger, and episodes of diagnosed depression than their employed counterparts."

"Employed moms spend fewer hours per week with their children than stay-at-home mothers, but they spend more time with their children than homemakers did in 1965!"

"A recent multiyear study by the sociologists Adrianne Frech and Sarah Damaske found that women who worked full time following the birth of their first child had better mental and physical health at age 40 than women who had not worked for pay."

No links are provided to the sources of these claims, and I, for one, am not about to start digging up and analyzing data from over 50 years of studies. Because, aside from agreeing that working mothers should not, after all this time, still be seeking validation for their "choices," I don't quite understand why we need to be comparing them, by going head to head with stay-at-home-moms, in a competition of who enjoys themselves more, in order to get them the support they need.
The very fact that an article claiming working moms are less depressed and angry than moms who stay at home has the word "triumph" in the title, is more than a little unsettling to me.
What, if it is true, is so triumphant about the thought of millions of depressed and angry stay-at-home mothers?
Obviously the well-being of mothers is worth talking about. Whether moms work or not, the topic of mama rage, anti depressants, and moms self-medicating with wine comes up an awful lot on mom forums. As a mom who's resorted to Prozac more than once, I am personally curious about what's driving this perpetual unease and how it relates to stress and motherhood.
I would also love to know if the well-being of fathers is being so closely scrutinized and if, with the everpresent shifting of the tides, it ever will.
Lastly, if it's true that all these moms are so miserable at home, how can we better support them? How can we better support all mothers? That to me is just as important as making sure moms who work are seeing improved family leave policies.
Do you believe working moms are "happier" than SAHMs? If so, do you have any insight as to why that might be? Is it as simple as getting respect, and external reward, for your time and energy?