Birth Order Effects Exist and Are Strong
Speaking informally, but as a professional analyst here who is unfortunately at a computer with no analysis tools setup :(  but who has worked with his fair share of social, survey and demographic information.<p>Check your data.  My first gut-instinct on seeing data like that is not &quot;Eureka - i found a huge first-birth-order effect&quot;, but rather &quot;huh...looks wrong, I wonder why that is?&quot;.<p>What do I mean &quot;looks wrong&quot;?  Keep in mind I have no real &quot;pony in the race&quot; on first-born effects (although I admit I am one), but intuitively it &quot;doesn&#x27;t look like real social data looks like&quot;.<p>The effect is too big.  Its too loud.  Its too constant.  Its too nice.  Something else is driving it and its your job to find out what...<p>The first place I&#x27;d look is data quality.  Have you proven your data got into your final histogram data set right?  I had problems with your csv opening it in libre office...looks like it interpreted it as having some value migration between the variable in whatever you did.  Could be my computer, but in my experience, if there&#x27;s a problem with the csv, these kind of things correlate with other mistakes.<p>The second place i&#x27;d look is survey design: was there a default option, are people clicking through by default, are the response frequencies of other variables lining up as would be expected (gender, birthdays, that kind of thing).  If not, why not?<p>If you&#x27;ve crossed and ticked all those, only after that would I start the analysis...but again, a pattern and effect of that size and consistency is something that you should default to needing explained in the data. Reach the conclusion that its real only after you&#x27;ve eliminated everything else.