This post follows on from my previous post detailing some areas where I was unable to reproduce Scott’s analysis of how the age gap between siblings modifies the SSC Birth order effect. I suggest you read that post first but here’s the summary:

I attempted to reproduce Scott’s analysis of Birth order effect vs Age gap. I found that:

There appeared to be an error in graphs 2 & 3 where people with one sibling were counted when they shouldn’t have been (graph 2) or were counted twice (graph 3)

Comparing oldest children to youngest children causes a bias in the results which can be prevented by comparing oldest children to 2nd oldest children

I was unable to reproduce Scott’s result on people reporting 0 year age gap – I get a non-significant 58% older siblings compared to Scott’s 70%. I was unable to discover the cause of the difference.

Summary

I reanalysed how sibling age gap modifies the SSC birth order effect. I found that:

The birth order effect is relatively steady for the first 4-8 years of age gap at about 70% respondents being the firstborn vs secondborn. For larger age gaps the effect reduces. There is insufficient evidence to conclude how long this reduction takes or whether the effect is completely removed at very large age gaps.

2 other trends were noted in the data but evidence for them was not strong:

The reduction may not be the same (or might disappear) for larger families
Birth order effect may be lower at 1 year age gap vs 2-7 year age gap

Considering competing theories on the cause of the Birth order effect, two theories fit the data well:

Intra-family dynamics
Decreased parental investment

And three theories fit the data poorly:

Changed parental strategies
Maternal antibodies
Maternal vitamin deficiencies

Introduction

The original reason for me looking at this data was to analyse whether the data support a sudden drop between years 7 and 8 or whether there is an alternative explanation which fits the data.

I will note here that I’m not a trained statistician and am using this as practice of Bayesian model comparison, inspired by johnswentworth’s recent model comparison sequence. I’d say I’m 80% confident in my broad conclusions, less so in the specifics - I'd be fairly confident there are a couple of errors lurking in here somewhere.

Analysis: All family sizes combined

Is there a sudden drop after 7 years?

Getting back to the data, here’s the result th...

Birth Order Effect

Birth Order Effect