For years tech companies and other open-plan evangelists have argued that despite employees' grumblings about privacy, open-plan offices have one killer selling point -- they spur employees to interact more, sparking fresh ideas and boosting collaboration.

It's a compelling story (one that also sounds nicer as a justification than lower real estate costs), but many people who have actually tried to talk to a colleague in a wide-open, too-quiet office have been suspicious of the claim.

Now science has backed up their hunch. If you've long felt open-plan offices were a collaboration killer, a new Harvard study proves you were right all along.

More email, less conversation.

The design of the research was simple but incredibly clever. Study two Fortune 500 companies planning to make a switch to open-plan offices and compare how employees interact both before and after the new office design.

To do this, Harvard researchers Ethan Bernstein and Stephen Turban had 150 participating employees wear a gizmo called a sociometric badge. For three weeks before and after the redesign it recorded wearers' movement, location, posture and, via infrared and sound sensors, their every conversation with colleagues. The researchers also reviewed the number of text messages and emails subjects sent during the test period.

The results have just been published in the journal Philosophical Transactions of the Royal Society B. What did they show? In short, as walls came down, so did the number of interactions among co-workers. Simultaneously, the number of emails and text messages shot up.

"Overall, face-to-face time decreased by around 70 percent across the participating employees, on average, with email use increasing by between 22 percent and 50 percent (depending on the estimation method used)," says the British Psychological Society Research Digest blog, summing up the results.

On his blog, computer science professor and Deep Work author Cal Newport put those percentages into startling perspective. "To make these numbers concrete: In the 15 days before the office redesign, participants accumulated an average of around 5.8 hours of face-to-face interaction per person per day. After the switch to the open layout, the same participants dropped to around 1.7 hours of face-to-face interaction per day," he writes. That's an astonishing four hours less of collaboration per day.

The study co-authors were blunt in their assessment of the data: "Rather than prompting increasingly vibrant face-to-face collaboration, open architecture appeared to trigger a natural human response to socially withdraw from officemates and interact instead over email and IM."

Exploding the fake case for open-plan offices.

But, you might object, maybe all that interaction before was time-wasting chitchat. Maybe open-plan offices eliminate the privacy necessary for slacking, pushing people to talk less frequently but more substantively. Nice try, but the data showed it wasn't just the quantity of collaboration that went down, but the quality too.

"[In] an internal and confidential management review, [the company's] executives reported to us qualitatively that productivity, as defined by the metrics used by their internal performance management system, had declined after the redesign to eliminate spatial boundaries," note the study authors.

"Well, I could have told you that," many will respond. But while those who have been tormented by the many interruptions and conversation-chilling effects of open-plan offices have suspected as much, this study puts a final nail in the coffin of the idea that open-plan offices are about anything besides saving money or surveilling employees.

If someone tries to tell you otherwise, point them to this study.