The word "smart" appears in the title of the book being publicised, and in the title of the article, but appears nowhere else in the article. The comments here, and the title of the article, are quite misleading about the content of the article.

"Smart" has little or nothing to do with it. It so happens that whether you're smart or not is not a predictor of your happiness. The article might as well be called "Why so many people born on Tuesdays aren't happy."