Those scientists were studying the wrong ladies. If a guy decides to be a Debi Downer I don't wanna be around him. For me, when a guy is happy, I'd much rather be around him. Mainly because moods can be contagious for me. If someone is sad or upset I'm going to feel it and I'm going to feel really bad. Why would I decide to date a person like that?? If someone is happy, chances are, I'll feel happier, too.
I think the point was more about brooding, and these women were likely single. This is the thing behind it.
A single woman will look at a really happy guy and she'll be like 'he's so happy, he must have everything he needs', and this will play on a person's own security. 'If he's got everything he needs, he'll hardly need me'. Not only that, happiness seems to indicate satisfaction and contenment. It seems to me that women want men to not simply be content, but be 'hungry'.
If a guy looks brooding, or ashamed and slightly lacking somehow, a woman might also think 'I can fill that gap'. It's the whole 'I can change him, I can fill the hole in him' thing. And it's such a cliche but it's true.
And remember, too, this is based on still images, and basic first attraction, 'would I sleep with this guy' instincts. Prideful expressions or brooding expressions are supposed to be synonymous (apparently) with strength, power, fitness and fighting ability. (I know, it all sounds a bit animalistic and stupid, but there's a reason for this).
The theory is that men who look tough and arrogant will be more dominant, thus go further in whatever area of life, and this is supposed to be attractive because such people will be able to provide more money, comfort and status to their family, and if nothing else, men like this tend to care less what women think thus they are more likely to be confident leaders in lots of aspects of the partnership and in life.
There's also the thing about, if the dad is confident, cocky, arrogant and powerful, then the son will have similar traits and also do well for himself. He'll make lots of babies with lots of women and further the lineage.
In reality, although people don't like to admit it, attractiveness is most often initially based on pure sexual instinct.
The problem is, we live in a world where the cognitive reasoning like those mentioned above perpetuate the negative aspects of human procreation and sexual relations. Men who are arrogant, self-obsessed, stubborn, recklessly-driven, and really not good partners, are the ones that most (not all) women would like to father their children.
The other kind of guy - sensible, caring, affectionate, warm, smiley - makes a great relationship partner, but to many women lacks power and status.
In essence, it's a bit of a catch-22 for the women out there cause people often ask 'if you don't like coming second to a guy's ego, then why do you date jerks'? The answer really, lies in the fact that the world may not always be as lawful and peaceful as it is, and if that day ever comes in a woman's lifetime, the cocky, arrogant guy will fare a lot better in protecting and furthering the species.