Participants in the experiment played a basic economics game: Let's say Mary and Sally are playing. Both are given $30 to start with. Mary is told she can "invest" as much as she wants with Sally by giving that money to Sally. However much she gives to Sally is automatically tripled, and then Sally can decide how much to give back to Mary as a dividend. So let's say Mary gives Sally $10. That amount is then tripled, so Sally ends up with $30 + $10*3 = $60, while poor Mary only has $20 left. This doesn't seem very fair, so Sally is allowed to give any amount of money back to Mary.
Mary, then, has a difficult choice. She earns the most money if she gives all $30 to Sally, and then Sally gives the entire investment ($90) back. However, she also risks Sally absconding with the money, leaving Sally with $120 and Mary with nothing. So Mary's safest choice is to give Sally nothing.
So what happens?
The results of the study were that if Mary was a first-born, she gave $3.7 less money to Sally than if she was a latter-born (actually, the game used "monetary units," not dollars, so it's not clear how this translates). So first-borns trust less. Interestingly, if Sally was a first-born, she was also returned less money Mary, meaning first-borns reciprocate less as well.
It's not entirely clear that these are two separate effects. If Mary was predicting Sally's behavior by thinking about what she herself would do in Sally's shoes, you would expect first-borns to give less money (since they themselves would return less of it) and for latter-borns to give more (since they themselves would return more).
One nice aspect of this study is that they controlled for the fact that latter-borns tend to come from larger families (each family has only one first-born but potentially many latter-borns). This is unfortunately not something that birth-order researchers typically do, which is problematic since people from smaller and larger families differ in many ways (including parental income, education, etc.).
Why are first-borns so mean?
It's interesting that people differ based on birth-order, but I think what most people really want to know is why. Why were first-borns less trusting and why did they reciprocate less?
The authors didn't really address this question, but their data do suggest one possibility. They report that if Mary gave Sally $10, Sally gave, on average, $18 back, leaving Mary with $38 and Sally with $42.
Now, let's let Jenny and Susan play. Jenny decides to give Susan $30 -- all her money! On average, Susan would give $43 back. This is more money than Sally gave back, but keep in mind that Jenny also gave up more money to start with, so in the end Jenny walks home with $43 -- which is hardly better than the $48 Mary got. On the other hand, Susan cleans up, netting $77.
It's true that Jenny's generosity did get her an extra $5...but it also got Susan an extra $25, which she chose not to share. Since this unfairness tends to rankle people, Jenny may in fact be more unhappy at the end of the game than is Mary, despite the extra $5 (obviously, Susan would feel pretty good about the outcome).
Getting back to birth-order, first-borns were less trusting -- that is, they invested less money. As we see from the analysis above, they were making the right decision; investing wasn't really going to pay off for them. They also returned less money from the investment, which may show that they are "bad reciprocators," or it might just show that after years of dealing with their younger siblings, they've come to the hard truth: people -- even younger siblings -- are greedy jerks. They returned about as much money as they would expect to get were the positions reversed.
Would the world be better off without first-borns?
For anyone who was wondering whether the game would turn out better if only latter-borns played, even though latter-borns do return more money from the investment, they don't return much. Invest $30 with a latter-born, and in the end you'll have $47, and s/he will have $73 -- better than if you invest with a first-born ($36.7 and $83.3), but still unfair.
Illustration from the Creative Commons