This is actually a remarkable news article because it does not confuse correlation and causation. It notes the association between sexual activity frequency and income, and then speculates about "third variable" factors that might be contributing to the association.
I should warn against dismissing studies that are "just" correlational. The correlation between Conscientiousness scores and job performance enables employers to select significantly superior employees. The correlation between smoking and lung cancer is the foundation of the FDA warning labels. There's a correlation between surgeons' blood alcohol levels and the risk of complications during a procedure. I wouldn't let some drunk guy operate on me just because "correlation doesn't prove causation."
On a side note, it's kind of sad that so many economists find their own field so boring that they busy themselves conducting psychological research.
Employees that have sex more than four times a week receive 5% higher wages, according to an academic paper by Nick Drydakis, a senior lecturer in economics at Anglia Ruskin University in Cambridge, England. Conversely, those who don’t have any sexual activity earn 3% less in wages than those who are sexually active, the study — published by the Institute for the Study of Labor in Bonn, Germany — concluded. Similarly, a Brazilian study published in 2009 found a positive correlation between sexual frequency and wages for Brazilian employees. This may be little more than a correlation, but based on these findings, Drydakis says, “it seems that sexual activity may be of interest to economists.”
Why the bigger paychecks? Sexually active people may exhibit more attributes that are prized in the workplace, experts say. “Both sexual activity and higher wages convey a feeling of higher self-esteem and self-confidence, which attracts more sexual partners and more work opportunities,” says Carole Lieberman, a psychiatrist in Beverly Hills. “Put succinctly: Everyone loves a winner.” This also ties in with long-running theories that attractive people earn more money, she says. In fact, so-called beautiful people are likely to earn 3% to 4% more than plainer folk, according to “Beauty Pays: Why Attractive People Are More Successful,” by Daniel Hamermesh, an economics professor at the University of Texas in Austin.
Another possible explanation for the connection: Those who are more sexually active may simply be in better shape emotionally and physically, which could make them more amiable, productive and creative employees. “This actually doesn’t surprise me, if sexual activity is just one more indicator of general well-being,” says Tina Lowrey, professor of marketing at Hautes Études Commerciales de Paris, a business college in France. Drydakis agrees, adding that increased sexual activity could be a key indicator of good health. “Medical and psychological literature suggest that sexual activity is associated with good health, endurance, mental well-being, mental capacities and dietary habits,” he says.
One caveat, however: The positive correlation between sex and higher wages could also work both ways. That is, higher wages may encourage some to adopt more sexually active lives, Drydakis says. “They may increase the value and attractiveness of a person on the dating market.” Some psychotherapists say there’s a connection. “The more success the individual experiences, the higher his libido rises,” says Fran Walfish, a therapist in Beverly Hills. And, she says, less money could also mean less sex. “I am currently treating two men whose incomes have dramatically decreased because of the poor economy,” she says. “Both men have reported a significant decrease in their sexual desire and sexual activity.”
For more on why libido might drop after losing a job or being passed over for promotion, check out this paper on the Social Competition Theory of Depression (Price et al., 1994).