Percentage of Firstborn Children

At a talk this weekend on the Passover fesitval, it was mentioned that one of the things remembered at the feast is the sparing of the firstborn Hebrew children by the Angel of Death just before the exodus - the story.

Anyway, the speaker asked all the firstborn children to put thier hands up, and it was well over half the people in the room - which surprised me. Then I thought about it for a bit and realised that one should expect that firstborn children would make up the largest group of children (compared with second/third/etc born), as every family with children has a first born, but not all have second/third/etc. And that got me wondering what the actual percentages would be, but I cannot find any statistics showing the distribution of family sizes…

Or is there another way of approaching the problem?

Grim

Here’s a pdf file showing percentage of first births in Masschusetts in 1999. Scroll down to page 27.

http://www.state.ma.us/dph/bhsre/birth/99/birth99d.pdf

Note this is not the percent of first borns in the total population, only the percent within that’s year’s births. Other states are likely to provide similar data.

I meant to include the percentage shown there. - 44.3%.

Proportion of first borns
= #FB/#children
= #families/#children
= 1/(#children/#families)

= 1/(mean no. children per family)

The mean (average) no. of children is just under 2 IIRC (easy to look up) so the proportion of first born children should be just over half.

Oops, this doesn’t agree with aahala. Maybe Mass. is atypical? Or I boobed the calculation.

Shade, I was going to make a similar comment about your equations. But I think your input percentages are a bit off:

According to this(pdf) the total FERTILITY rate indeed sometimes is less than 2, however, the average no. of children per family is slightly greater than 2: some women do not have families.

Slightly above 2 (2.1) would, of course, result in a close approximation of the 44.5% statistic.

I don’t know how to reconcile that with the OP’s experience; after all, it is only recently that family sizes have even gotten THAT low, according to the above cite, during the baby boom average family sizes were above 3, and one would expect some older people in the audience.

Waitasec, that was for America, I’m not sure what the stats are like in London, but I still would think they would be above an average of 2 (resulting in a less than %50 firstborn rate.) After all, I learned that some Scandinavian countries indeed have a average family size below 2, and I would have thought that England would have been included in that (not that they’re Scan), if they were also that way.

Which doesn’t really answer the question. There’s no logical reason for a majority of children born within a year to be firstborn.

There is, however, a logical reason for this to happen with a general population.

Errr… oops. That part isn’t necessarily right. I was assuming that a sufficient portion of families stop at 2.

Carry on.

Engywook, I’m not sure I follow your logic.

If the 44% is representative for firstborn births in a particular year(and I’m not claiming it is), then the percent of firstborns in the entire population is likely to be less not greater. Some first mothers in any given year will have additional children.

Ummmm… that’s because I must concede that it’s wrong… :smack: I could go into detail about what I was thinking, but why waste the effort over an error.

Out of any sample, the number of firstborn children would most likely hold a plurality… not a majority.

Off I skulk to a dark place…

Just wanted you all to know that I find the question interesting and despite the smack-head smilies, I’m enjoying your attempts to find the answer.

~Ellen, cheerleader for GQ answerers.

Thanks, Ludovic.:smack: OK, so this agrees with aahala now.

Don’t know what’d account for the OP’s experience, then. Maybe more firstborns go to these sort of events? Or people just like putting their hands up. But I’m leaving GQ with this.

I was wondering about that as well, since we were all evangelical Christians of one sort or another, and I wouldn’t be surprised to find that oldest/firstborn children have a tendancy to follow in the faiths of thier parents…

Thanks for the answers folks, any more contributions welcomed.

Grim

The data seems to suggest numbers below 50%. However, it is conceivable that firstborns suffer a lower infant and adolescent mortality rate than second- and thirdborns. If so, this could kick the adult firstborn percentage up past 50%. I seriously doubt it, though. I think it’s more likely that it just looked like much more than half.

Oh, it was well over half, but a sample set of 25 Christians meeting in South London means nothing…

Grim

25? Yeah, that’s not very significant. Just lucky, I guess.

If the average number of children per family stayed the same for a couple of generations, then the percentage of firstborn(ignoring child deaths), would be very close to the recriprocal. Average children per family of 2 means 50%, 3 33.3% and so forth.

If the average declines consistently over time, then the recriprocal would overstate the percentage as parents themselves would not be reflected in the current year. And vice versa.

Any group you talk to about this may be unrepresentive of the true population wide percentage. For example, if everyone in the US meeting were aged 56, 57 or 58, it would mean they were born just after WWII and such a group would likely have a very high firstborn element due to the that.

Grimpixie, you’ve overlooked one minor flaw. Not every family with children has a firstborn. In some families (and I would imagine that in ancient societies with higher infant mortality rates this would occur more often) the firstborn may already be dead from an entirely different cause. Thus, not necessarily every family in Egypt had a firstborn at the time of the tenth plague.

In any event, Jewish tradition teaches that if a household had no firstborn, then the oldest in the household died that night (as per Exodus 12:30).

However, I don’t know what the birthrate was in ancient Egypt, so I don’t really have an answer to your question.

Zev Steinhardt