It's self reported. N=41 and even in the self reporting they give that men that were in a relationship had few interactions with people of the opposite sex.
Twenty-seven participants reported being single at the start of the study
Given the rarity of DSI occurrences in partnered men, and the expectation that mate attraction efforts are more important for single men, we tested whether DSI moderated the relationship between testosterone and courtship efforts specifically in single participants.
So only the data of 27 persons were relevant for most of the conclusions of the study.
It was a study of one month:
we collected daily measures of salivary testosterone for one month (31 days), as well as self-reports of sexual desire and other states or events relevant to mating effort on days corresponding to the hormone measures.
It looks like not terribly significant a study. Don't read too much into it.
People see a sample size of 30 and assume it's instantly a bad study. A small sample size can still have considerable power depending on the experimental design and assuming that the sample is actually random across the target population. At the very least, it's information that can be used to guide the experimental design of future studies.
I have kleinfelter syndrome... When I started TRT and they hadn't dialed in my dosage. Ho-boy is this comment accurate.
Edit: Now that I think back it wasn't the testosterone that sent my libido into overdrive it was the fact that my body was converting the excess testosterone into estrogen. Because when they started me on Tamoxifen it managed to reign my libido in, and when they switched me to Anastrazole it brought my libido down to normal levels. "Normal" being equivalent to that of a teenager, my wife comes to bed only after I've gone to sleep and she erects a pillow wall between us.
Steroids usually decrease libido, so I doubt they would tell you otherwise, and if they did, they would be talking testosterone esters, and not a natural steroid hormone.