Birth Order and Success: Implications for Parenting

Yet another study has suggested that birth order predicts adult success. This time the analyses are based on data collected by the dating website PlentyOfFish (POF). Researches at POF examined data trends among 7.6 million singles (in the US and Canada, between the ages of 25-45) using their services – so even though this is not inherently a representative sample (by epidemiological standards) it is still a very large sampling of adults and worthy of consideration. To this point, POF offers analyses on relationship success as well as the more standard benchmarks of education and income. Here we’ll summarize the study findings, caveats of the research, and implications for parenting … and speak briefly to the relative status of singletons.


The primary findings are as follows:

FIRST BORN CHILDREN HAVE ADVANTAGES: According to Sarah Gooding, Media Spokesperson, PlentyOfFish, “This PlentyOfFish study suggests that firstborns are more likely to succeed in life – in the areas of education, income and love. For example, the firstborn male of four is 13% more likely to find a relationship compared to the average male user from the study.”

MIDDLE BORN CHILDREN ARE LEAST LIKELY TO FIND RELATIONSHIP SUCCESS: Here relationship success was defined as leaving the POF site in a relationship. Gooding summarizes the findings as follows: “Regardless of family size, middle children are the least likely to find a relationship.”

YOUNGEST CHILDREN EXPERIENCE DISADVANTAGES: They are less likely to pursue higher education and (perhaps as a result) tend to make less money. Gooding explains: “As the family size increases, the likelihood that the youngest of the family will pursue higher education or make an income over $75K/year decreases. Our study found in a family with two children, the youngest is 3% more likely to pursue a PhD. In a family of three children, the youngest is 13% less likely to pursue a PhD, and this trend continues in a family of four children where the youngest is 17% less likely to pursue a PhD.”

GENDER MAY MATTER: Birth order effects were more pronounced for males in the POF study, as discussed by Gooding: “Our findings suggest that the female youngest of two is more likely than their male counterparts to pursue higher education. The females youngest of two is up to 9% more likely to pursue a Masters degree or PhD. In comparison, the male youngest of two is only up to 3% more likely to pursue a Masters or PhD.”

What You Need to Know About Birth Order
What You Need to Know About Birth Order
What You Need to Know About Birth Order


Let’s start with the obvious. We have all heard that correlation does not imply causation. If you takes Stats 101, you’ll be taught this principle and come away thinking survey research doesn’t “prove” anything. While that’s true in the technical sense, the fact is we rely on survey data to reveal naturalistic associations that have some real-world meaning (as opposed to experimental designs which may artificially control a lot of variables and hence suffer from a lack of “ecological validity” – meaning direct application to the naturalistic, uncontrolled world we live in). So, yes, this study relies simply on statistical associations, but that doesn’t mean we can’t try to abstract some meaningful principles from them.

I’ve also already stated that this is not a representative sample – so it’s been duly noted (twice here!) that this offers a somewhat skewed sampling perspective. Again, though, this does not mean that the data aren’t interesting, especially given the size of the sample (and the fact that a fair number of adults use online dating services).

What I’d like to bring attention to is the good and bad of working with very large samples like the one gathered by POF. The “good” is pretty clear – the larger the sample, the more confidence we have in the statistical parameters. It’s better to sample around 8 million people and draw statistical inferences than it is to sample around 8 people. You get tighter statistical estimates which increase the “statistical” confidence in those estimates.

But there is something of a down side of large sampling, if a statistical principle is ignored. Because of this statistical confidence, it’s easier to find a statistically significant result – which simply indicates that there is an effect (e.g., a correlation that is not “zero”) rather than signifying the magnitude of that effect. Thus significance should not be confused with the size of the statistical effect – very small effects can be significant when looking at large data sets.

That’s why it’s important to look at the effect size of studies like this one. I’d (informally) characterize the effects here as “modest” in the statistical sense, which would be consistent with lots of other studies on birth order. Birth order seems to have a systematic effect on adult education, income, and (especially in this study) one indicator of relationship success. But it’s important to keep in mind that this effect is modest – meaning that lots of other factors come into play. Put another way, don’t be surprised if your own informal sampling of people you know indicates that there are some youngest siblings who achieved the highest level of education in their family, and some middle children who had the most relationship success, and some first born children who don’t make a lot of money and aren’t in a relationship. It’s the overall patterning in the population studied that comes into play in large studies like the one conducted by POF.


Full disclosure: I’ve never been especially interested in birth order effects (and I’ve been doing research on siblings for, well, a lot of years now). I’ve always felt that birth order was background noise – with a little signal – that got way too much attention in the popular press as compared to the actual processes that go into parenting multiple children. But I’m softening a little bit, in part because birth order effects get replicated, but more so because there is better thinking about them now in terms of parenting.

The most clear cut implication for parenting is that younger children, especially in larger families, may not be getting the same kind of focused parenting that their older siblings received. We know that parents will say that by the time the last one comes around, they are more relaxed with him or her compared to the older siblings. Some of that reflects experience as a parent. But it’s important to keep in mind that the bar shouldn’t be lowered in the case of academic standards – research using national data bases suggest that, in terms of academics, parents may spend less time monitoring their youngest child’s homework, which may account for some of the scholastic effects of birth order.

What about middle children and relationships? One speculation can be offered – and bear in mind it is indeed conjecture. We often hear of the Middle Child Syndrome – being stuck in the middle of the presumed favorites, the oldest and the youngest. Perhaps there is something in this relationship dynamic that impacts (to a degree) the ability of middle children to find success in relationships. Again, this is simply an idea to think about – and perhaps one worthy of more dedicated research.


One last point. Only children also seemed to be less inclined to be successful in the POF study. Morgan Cabot, Research Analyst, PlentyOfFish, offers the following summary and interpretation: “Our findings suggest that single children are much less likely to pursue higher education and earn an income over $50K/year. This finding was the most surprising as the general assumption is that single children receive more attention and financial support from their parents, making it easier for them to succeed in life. There are likely many factors influencing these results. One theory is that the very fact that single children don’t have a sibling (to set an example for or to compete with) leads to a lower drive to succeed in the areas of education and income.”

Perhaps the best takeaway in terms of parenting an only child is the same one that applies to youngest siblings: make sure the bar is set high enough in terms of expectations about achievement.

What You Need to Know About Your Only Child
What You Need to Know About Your Only Child
What You Need to Know About Your Only Child

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