Meaningful conversations are incredibly powerful. They ease our loneliness, spur thinking, and according to research, boost happiness. In business they are a wellspring of creativity and decision-making wisdom.

But while great conversation is incredibly valuable, it's also elusive. Sometimes a group just seems to kindle and thoughtful discussion flares. Sometimes you trudge through an evening scrambling from one topic of small talk to another.

Some of this is down to the personal chemistry (and conversational skill) of participants, but as scientists are beginning to learn, a little social engineering can help you increase your chances of having a really great conversation. The first place to start is with the size of your guest list.

When it comes to great conversation, less is more

You've probably noticed this phenomenon as a guest: sometimes you attend a dinner party or meeting packed with lots of smart, interesting people only to find the conversation never gets off the ground. The problem, according to new research out of Arizona State University and Oxford, may not be that there are too few clever brains in the room. But instead that there are too many.

The research kicked off with an observation. When the researchers involved were walking around campus looking at knots of students deep in conversation, they rarely numbered more than four. Was there some magic number of participants for deep, nourishing conversations, they wondered?

To try to probe that question, the researchers turned to an unexpected source -- the plays of William Shakespeare. The great playwright may be far away from us in time, style, and subject matter (unless any of you out there spent much time plotting to kill the king), but apparently the conversational "rule of four" still held in the 16th century. Throughout the ten plays the team analyzed every meaningful conversation had four or fewer participants. 

Why this hard limit of four? The researchers theorize that if might have to do with our ability to imagine what other people are feeling and thinking, what psychologists call "theory of mind." Doing this effectively, as you've probably noticed in your own life, takes a lot of mental bandwidth. The scientists suspect we simply don't have the capacity to track more than four people's interior states and what they think about a discussion topic at a time.

Remember this next time you host a dinner party or meeting. 

While this is still a theory, it's an interesting one for anyone who is hoping to host a gathering that will feature thoughtful discussion. While bigger groups can break off into smaller subgroups for deeper conversation, it's probably impossible for, say, five or six people around a table to really lose themselves in meaningful exchange. Remember that next time you're hosting a party -- or planning a meeting where you want to dig deeply into a complex issue.

Paying attention to the setting may also help, suggests psychologist Frank McAndrew in his take on these new findings for Psychology Today. "Other situational factors, such as the arrangement of furniture, can influence the ease of conversations. For example, although side-by-side seating connotes intimacy, it does not seem to be the preferred arrangement for talking," he writes. When people are left to choose their positioning for themselves, they tend to sit face to face around five feet from each other.

"In other words, if it is good conversation that you are after at a party, stay away from the couches," he concludes.