SHANKAR VEDANTAM, HOST: This is HIDDEN BRAIN. I'm Shankar Vedantam. People in romantic relationships often share things in common, a love of the same sports team, maybe the same religion, sometimes a common profession. But for some couples, the similarities go further.UNIDENTIFIED MAN #1: The girl that I'm dating now, we have the same birthday, which is kind of cool.UNIDENTIFIED WOMAN #1: My fiance, Israel (ph), and I have the same birthday, which is something we discovered on our second date.VEDANTAM: You could say this isn't so surprising. There are lots of people in the world, and sooner or later, some people are going to get together with others who happen to have the same birthday. But there's evidence this may be more than just a coincidence. Having a common birthday seems to actually draw people closer to one another.UNIDENTIFIED WOMAN #2: When I first went to a new hairstylist, we found out that we have the same birthday, and we call each other birthday twins, and I wound up going to that hair stylist for years, even after she switched salons, even after we moved. I would drive 40 minutes to go get my hair cut by her.VEDANTAM: Some people see so much significance in shared birthdays, or even shared numbers in two birthdays, that they select dates for important events based on those patterns.UNIDENTIFIED WOMAN #3: My birthday is August 2, and my husband Blake's (ph) birthday is August 9, and we got married on August 29.UNIDENTIFIED MAN #2: My wife and I share the same birthday, July 6, and we are also married on that day.JANETTA CRAVENS: My birthday is October 27, and with my first husband, his birthday was September 27, and we had the opportunity to get married on August 27. It was a no-brainer. We decided to do that.VEDANTAM: The woman you just heard, Janetta Cravens (ph) from Oklahoma City, says her love for people with the same birthday extends beyond her love life.CRAVENS: October 27 is also Teddy Roosevelt's birthday, and so I've always felt a little affinity for him and for his leadership.VEDANTAM: Shared birthdays, in fact, aren't the only thing that draw people closer to one another. Janetta also finds herself drawn to people who happen to have the same name.CRAVENS: When I meet another Janetta, there's an automatic like, oh, my gosh, I can't believe we have the same name. And then if it happens to be that we have the same spelling - and I'll tell you, I've only met two people who have the same spelling as me, but I can tell you who they are and I felt an automatic kinship.(SOUNDBITE OF MUSIC)VEDANTAM: These unexpected connections with strangers can be lovely moments. But there's something else at work, too, something we may not be aware of, something that affects all of us. In Greek mythology, the hunter Narcissus was so enamored of his own beauty that he fell in love with a reflection of himself. Modern psychology shows that we all have a little bit of Narcissus in us. Most of us like people who remind us of ourselves, whether that's someone else with the same name or someone with the same birthday. Most of the time, such self-love is amusing and harmless, maybe even beneficial, a sign of good self-esteem. But there are times when falling in love with ourselves or with people who remind us of ourselves can be a real problem. This week on HIDDEN BRAIN, what we are calling the Narcissus effect.(SOUNDBITE OF MUSIC)VEDANTAM: Researchers have long observed a tendency for people to be drawn to others who are like them in some way. When we hear someone else has the same birthday or the same name, we feel a little tug of kinship. Some of us might even be drawn to move to states that sound familiar.BRETT PELHAM: There's at least a modest tendency for women named Georgia to gravitate toward Georgia, women named Virginia to gravitate toward Virginia, and the more closely the name resembles a state, the bigger the effect appears to be.VEDANTAM: This is Brett Pelham. He's a professor of psychology at Montgomery College in Maryland. He studies something called implicit egotism.PELHAM: Which is the idea that many biases are unconscious. And one very well-studied bias was egotism, valuing the self favorably, protecting the self and so forth. And so we simply got the idea that there are several different things that at least to some degree reflect a preference for the self and an attraction to things that resemble the self.VEDANTAM: Like having an affinity for someone with the same birthday or someone with the same name or even going into a profession that sounds like an echo of your name.PELHAM: So we originally looked at whether people named Dennis (ph) or Denise (ph) gravitate toward dentistry. But we learned pretty quickly...VEDANTAM: (Laughter).PELHAM: ...That it's really hard to get data on those things. So you - there aren't great directories of medical professionals. There is a guy named Abel who was able to document, and I was a reviewer on his paper and I...VEDANTAM: So now every time you say this, I'm going to jump in and say a guy named Abel (ph) who was able to do something.PELHAM: He was able to do a lot of things. He showed that people whose last name is doctor or whose last name is the word lawyer, the name Lawyer, gravitated toward those two professions.VEDANTAM: There are lots of other examples. Brett looked at a massive database of millions of Americans, names from the recently released 1940 census, to examine if there were broad patterns, like...PELHAM: Carpenters working in carpentry, bakers working as bakers, butchers working as butchers, miners working as miners, masons working as masons. So we looked at every surname there is - currently, the top 2,000 surnames - that happens to be a career name, and we looked at all of them. There are 11 of them that are pretty common. I just listed a few. And for every single surname in the 1940s census with something like a - I think it was 130 million people - we were able to show that for every single surname there was at least a weak tendency for people to gravitate toward careers that perfectly matched their last names.VEDANTAM: So the obvious thing to say, of course, is that the reason you have a slightly larger number of carpenters be Carpenters is that the name Carpenter probably originated from families who were in carpentry. And so there is some kind of ancestral connection to the profession that is driving both the names and the choices.PELHAM: Quite possibly, but if you do the math, you pretty quickly see it gets to be a pretty tiny percentage of people. So if you assume that even over 10 generations, there's a 50 percent chance that you did what your dad did and a 50 percent chance that he did what his grandfather did, which is probably higher than reality, you're talking about a probability of less than 1 in 1,000 already over 10 generations. In other words, surnames are so old and they change hands, you know, when a woman named Carpenter marries a guy named Farmer, for example, that that's really - it really just can't account for an effect anywhere near this magnitude.(SOUNDBITE OF MUSIC)VEDANTAM: Brett Pelham has looked at large data sets and public records like the Social Security Death Index or state marriage records, databases with information about millions of Americans, and he and his fellow researchers have seen all kinds of funny effects. Like, for example, one listener called in to say she and her husband were both born on the 25 day of different months. This isn't as uncommon as you might think. Beyond what you might expect by mere chance...PELHAM: There's about a 7 percent bias in that direction. So if you should have a thousand people doing that, you have 1,070 people doing that.VEDANTAM: If that doesn't sound like a lot, you're right. You should think of implicit egotism as a tiny, invisible nudge. It won't shape what everyone does all the time, but it does shape what some people do some of the time. It's when you multiply these small effects over hundreds of millions of people that you start to see lots of examples. Brett says that if you really love your birthday, the effect gets even larger.PELHAM: Our operational definition of really loving your birthday was getting married on your birthday number. So if you got married on the 13 of the month, you were quite a bit more likely to marry another person who also had the number 13 as his birthday number.VEDANTAM: Again, Brett calls this an implicit bias for a reason. You may not be consciously choosing things as important as your spouse or your profession or the place you call home based on arbitrary factors like your name or your birthday. He thinks the bias comes about simply because we like our names and our birthdays and have positive associations with them.PELHAM: And so you just associate your name with all the wonderful things that come along with that. And the best bit of evidence I have that I've never bothered to publish is there's one part of people's names they don't like that much, and that's their middle names. You know, and the joke about that is the sole purpose of a child's middle name is to know when he's really in trouble. And my additional comment is to know if he might be a serial killer someday, right?VEDANTAM: (Laughter).PELHAM: So the middle name is not nearly as loved as the first or the last name. People feel very ambivalent about their middle name.VEDANTAM: There can be other reasons for ambivalence. Sometimes we dislike it when another person has the same birthday. If you were born on April 20, you might hate the fact Hitler was born on that day too. Brett finds that if a consumer product happens to have your name on it but is poorly built, you are more likely to hate that product. People dislike seeing something inferior in the world that has their name on it.(SOUNDBITE OF MUSIC)VEDANTAM: As Brett and I were talking, I realized something a little uncomfortable - that I myself had fallen prey to implicit egotism.You know, as you're talking right now, I am realizing something about myself for the very first time. There is - there was a philosopher in India whose name was Shankara - my name - and he actually taught a philosophy that is known as Vedanta. And I've always been drawn to the fact that I find the philosophy of Vedanta to be very interesting, and the fact that Shankara taught that philosophy I thought was just charming. But as I'm listening to you say this, I'm realizing that this could just be implicit egotism.PELHAM: I've read his work very well, and it truly is wonderful. It's brilliant. So I think in this case, you made the objectively good decision. Yeah.VEDANTAM: (Laughter) At the same time, it does make you think, though.PELHAM: Absolutely, yes.VEDANTAM: Has this ever happened to you, Brett? Do you ever think about yourself and how implicit egotism is affecting you?PELHAM: We're getting a little personal here, but I can't resist being a little bit personal. I would say that probably the most dramatic example is that my son Matthew, his last name is Polan (ph). His mother's last name was Polan. My last name is Pelham. And I entered a relationship with her before I developed this theory (laughter) not after I developed the theory to validate the theory.VEDANTAM: So you're a Pelham and you got into a relationship with a woman named Polan?PELHAM: Polan - and we are very different in most other ways. That's about as much as I'll say about that (laughter).VEDANTAM: Are you saying that the names played some kind of a role?PELHAM: I have to think it did. I have to think - and we were from different backgrounds, different religions. I mean, we're - we come from very different worlds. Yeah, and yet, I was very attracted to her.VEDANTAM: So...PELHAM: And worse yet, she was attracted to me, right?VEDANTAM: Yeah (laughter). So I understand why you would sort of call this charming, and I think if you do this work for a while, there is a certain smile that comes to your face as you sort of look at these connections and you sort of see - you understand sort of the choices that human beings make. But I do think there's something disturbing about it because I think there is a very strong sense that I think most of us have. I have the sense, even though I've been covering the world of the unconscious mind for the last 10 years, I have the sense that my choices are deliberate choices, that I've thought about them. I'm actually making them intentional. I'm making them intentionally. And the idea that you're coming in and telling me that there are these hidden factors that come in and change how I think about myself, it is a disturbing idea.PELHAM: It is a little bit disturbing. And of course, most people do what you and I do, which is say, well, of course, that applies to the rest of the world, but I in particular would never fall prey to implicit racial bias or implicit egotism. So most of us do tend to sort of separate ourselves from even our own findings. But I've become, in the past decade or so, comfortable enough with the study that I have to admit that I really never know for sure exactly why I did something.I mean, sometimes I have a pretty good idea, but I don't - I certainly don't kid myself anymore to say that I even usually know why I do what I do. I mean, there's just too much research - and much of which you've documented and reported on - that show the biases we're completely unaware, nudge us, sometimes powerfully drive us in very particular directions. So I think I've just kind of let go of it. It was disturbing to me when I first began to study it - disturbing as well as delightful. And now it's become more delightful and less disturbing.(SOUNDBITE OF MUSIC)VEDANTAM: So perhaps you're wondering, how does any of this matter? Who cares if people born on the 23 of September marry those born on the 23 of July? Who cares if someone named Betsy Carpenter gravitates towards carpentry? Why does it matter if I prefer a philosopher whose name is similar to my own?PELHAM: It at least raises tough questions about the degree to which we have free will. So I think some people who have pretty negative reactions to this work are very threatened by it in the sense that it suggests that very important decisions have at least a little nudging influence based on things that you're completely unaware of. So if people are more likely than they should be by chance to marry another person who happens to share their birthday number, that's not an objectively great reason to get married. You should get married because you share values. You're both Republicans. You're both rabid Marxists.But learning that these subtle little influences can affect what you do, apparently, to some people is - I find it delightful. But some people find it pretty threatening. I don't think that it proves that there's no such thing as free will. But it - to me, it does suggest that we don't always have free will. Sometimes we make a decision for one reason that we've told ourself when really the more powerful underlying reason is something we could have never put our fingers on.VEDANTAM: And this isn't just a matter of being drawn to someone who shares your birthday. In general, we tend to prefer things and people who have something to do with us. And that can be a big problem.PELHAM: Absolutely. I do a little bit of research on social justice, and that's one of the things that concerns me most, is that we tend to focus on people who are more like us, who speak our language, who speak our idiom, who look like us, who worship like us. And we pay much less attention, sadly, to the problems of people who don't. On the other hand, I think this finding, like any finding in psychology or behavioral economics, can be used for good or evil. And it - the way it works in the real world can be used for good or bad.And so a great example that I perceive as at least a cousin of implicit egotism is a study I think was done by Eliot Smith and colleagues about 10 years ago. He looked at implicit racial bias. And he looked at implicit racial bias as a function of whether a person from a different ethnic group - I think he had whites and African-Americans, for example - had simply given you a gentle friendly touch on the shoulder. And if they had given you a friendly, gentle hello-how-are-you touch, that reduced their implicit racial bias.So to me, when another person becomes a part of you, even in a very tiny way - you play instrumental basketball with this person. And you didn't like his group. But now that you meet him and he's on your team, suddenly he becomes a part of you. His group becomes a part of you. And your stereotypes get softened and diminished a little bit.VEDANTAM: When we come back, we're going to look at how the Narcissus effect shapes another important part of your life. If you can fall in love with your name or your birthday, can't you also fall in love with your own ideas, your own work? Of course you can. Researchers call this the Ikea effect.DANIEL MOCHON: We come to overvalue the things that we have created ourselves.VEDANTAM: We'll also look at the implications this can have for things much bigger and much more complicated than Swedish furniture. Stay with us.(SOUNDBITE OF MUSIC)VEDANTAM: This is HIDDEN BRAIN. I'm Shankar Vedantam. The actress Amy Poehler once said, Ikea is Swedish for argument. Researcher Daniel Mochon has one explanation for why build-it-yourself furniture can cause so much friction between partners.MOCHON: Imagine that you built a table. Maybe it came out a little bit crooked. Probably your wife or your neighbor would see it for what it is, you know, probably a shoddy piece of workmanship. But to you, that table might seem really great because you're the one who created it. It is the fruit of your labor, and that is really the idea behind the Ikea effect. It is that we come to overvalue the things that we have created ourselves.VEDANTAM: Brett Pelham's research about our preference for people and places that are associated with us made me think about a related idea. The Narcissus effect also shows up in our preference for things that we make ourselves.MOCHON: The way people usually think about Ikea is that Ikea gives you good furniture for a low cost because they offload lots of the costs onto their consumer - the assembly costs. But in fact, we're in a sense challenging that idea and saying that there's actually psychological benefit behind this, that actually people might end up liking their furniture more because they built it. And so it's not so much a cost but a benefit that they get to build their own furniture.VEDANTAM: Daniel and his coauthors did a series of experiments to test this hypothesis. They brought volunteers into the lab and gave them either a Lego car preassembled or gave them Legos and instructions and told the participants to build a car themselves. Then they asked the volunteers, how much would you pay to keep your Lego car?MOCHON: And what we find is that the people who build their own Legos not only are willing to pay more to keep their assembled Legos. But also, when we asked them how proud they are of their own creations, they tend to be prouder of their Legos. And mind you, these are Legos that are designed to 5 to 7-year-olds. But nonetheless, there seems to be some competence, some pride associated with one's creation, even for basic things as building Legos.VEDANTAM: The researchers replicated the study using other products, like Ikea furniture, and the effect was the same. People who spent time and effort building something felt proud of what they had built, fell in love with it and were willing to pay lots of money to keep the things they'd built. From the perspective of a rational economist, this doesn't make much sense.MOCHON: The students might be willing to pay twice as much money to buy the exact same Lego car if they just finished building that Lego car than if the Lego car was given to them prebuilt.VEDANTAM: So why might people value something more after building it themselves compared to buying the same product made by someone else for half the cost?MOCHON: Our hypothesis was that people tend to use products to signal valued identities to both themselves and to others. And we know that an identity that people really care about is showing that they're competent. This is sort of one of the basics of human motivation. And so we hypothesized that people use self-made products as a way to signal competence to both themselves and to others.You know, having just built a table or having just built a bookcase, that bookcase - that's completed products - acts in a sense of a badge of my own personal competence. I completed it. Therefore, I know I'm a competent person. And moreover, I can display this product and signal that identity, this competent identity, to others. And so we hypothesized that it was these feelings of competence associated with the products that led to their increasing valuation.VEDANTAM: To test the theory that people's feelings of competence was behind the Ikea effect, the researchers had some participants think about other qualities they might value in themselves besides competence - things like honesty or intelligence or humor. Basically, the idea was if we make it less important for people to demonstrate competence, do they still overvalue their own creations?MOCHON: So we found that the Ikea effect disappeared when we did that manipulation. So once competence wasn't that important to people, people, our participants, no longer seemed to get much value out of creating their own products, again, suggesting that the reason why we tend to like our own creations is because we use them as a way to signal competence, both to ourselves and to others.VEDANTAM: Here's the flip side of that coin. You can make the Ikea effect stronger by getting people to question their competence. In one experiment, Daniel and his colleagues gave participants math problems to solve before asking them to build an Ikea project. If the problems were difficult, lots of people failed to solve them. Now volunteers became much more likely to want to demonstrate their competence through the Ikea building project.MOCHON: The participants who, at least temporarily, had their sense of competence threatened, who got the very difficult math questions, tended to be more willing to build their own product. So when we surveyed them and asked them, would you prefer to have an Ikea product that comes prebuilt or the exact same product that comes unbuilt and you would build it yourself, those who got difficult math problems and were feeling somewhat incompetent at the time seemed more willing. They were much more likely to want to build the product themselves and therefore, in a sense, restore their sense of competence through this activity.VEDANTAM: I mean, give me the numbers here. I mean, was this a big effect? Was this a small effect? I mean, what happens when people feel bad compared to when they don't feel bad?MOCHON: So what we found is that about a third of the people wanted to build the Ikea bookshelf in the control condition when they weren't made to feel bad. And this number went up to about 60 percent when they were made to feel bad - so significantly larger number of people wanted to do this.VEDANTAM: The Ikea effect and implicit egotism might seem at first blush to be interesting and amusing but not terribly significant. But the more I thought about this, the more I saw the potential implications. Just as we can be drawn to those who have the same birthday as us or the same name as us, we might also be more inclined to help people who look like us or sound like us or live near us.Let's hear a congressperson who's drafted a particular piece of legislation. Your commitment to that bill might outweigh its importance to the public. Or let's say you're a president who starts a war. After years of investing your time and effort to prosecute that war, you may find it difficult to accept evidence that you made a mistake.It's fine to gaze lovingly in the mirror and to feel invested in our own ideas. But like Narcissus discovered himself, falling in love with our own reflection can come at great peril.(SOUNDBITE OF MUSIC)VEDANTAM: This episode of HIDDEN BRAIN was produced by Maggie Penman and edited by Tara Boyle. Our staff includes Rhaina Cohen, Jenny Schmidt and Renee Klahr. Our unsung hero this week is Andy Huether. Andy's the audio engineer for podcasting here at NPR. He's always willing to pitch in to help. When we have questions about sound design or the right microphone to use or how to make our voices sound their best in the studio, Andy always has good advice for us. Thanks, Andy.You can follow us on Facebook, Twitter and Instagram and listen for my stories each week on your local public radio station. I'm Shankar Vedantam, and this is NPR. Transcript provided by NPR, Copyright NPR.