Nielsen says it finally has statistical proof that Twitter traffic sometimes drives TV ratings — and vice versa — but the research firm doesn’t know why some shows are more simpatico with social media than others.

On Tuesday, the New York-based Nielsen released the Twitter Causation Study, which analyzed minute-to-minute trends for live TV ratings and Tweets for 221 broadcast primetime program episodes using Nielsen’s SocialGuide.

The findings: Live TV ratings had a statistically significant impact in related tweets among 48% of the episodes sampled, and on the flip side the volume of tweets caused statistical lifts in live ratings among 29% of the eps.

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Why does some TV work better with Twitter — and how many tweets does it take to boost ratings? The short answer: Nielsen doesn’t know yet.

“This round of causation research was only looking to see if there’s a ‘there there’ with respect to tweets influencing ratings and ratings influencing tweets,” a Nielsen rep said. “Now that we’ve seen statistically significant evidence of this, the next wave of research will be around understanding the how/why.”

Clearly, genre plays a part: Tweets had the greatest impact on competitive reality TV skeins, influencing ratings changes in 44% of episodes. Meanwhile, comedy (37%) and sports (28%) genres also saw increased tune-in from tweets, while dramas were least affected (18%) by tweets during episodes.

“Using time series analysis, we saw a statistically significant causal influence indicating that a spike in TV ratings can increase the volume of Tweets, and, conversely, a spike in Tweets can increase tune-in,” Paul Donato, Nielsen’s chief research officer, said in announcing the study.

In March, Nielsen released research that analyzed more than 140 broadcast and cable programs, finding that Twitter volume on the day of broadcast was one of only three variables analyzed — along with prior-year ratings and advertising spend — that affected TV ratings in a statistically significant way.

Nielsen cut a pact with Twitter last December, under which the duo plan to introduce a Nielsen Twitter TV rating with the 2013-14 TV season.

The latest research results “substantiate what many of our TV partners have been telling us anecdotally for years: namely, that Twitter drives tune-in, especially for live, linear television programming,” Twitter chief operating officer Ali Rowghani said in statement.