I'd have preferred if Scott posted an encrypted timestamped message on Pastebin before the survey (to register his intended experiments), and released the keys after the survey; "this could skew your results please don't read" is not particularly well controlled.

You don't really need to encrypt - just one-way hash. (Though if you count that as 'encryption' for this purpose, okay.)

The prevalence of older siblings could be because of other reasons perhaps? Only people born with a certain period of time are likely to be interested in transhumanism and of those more are older siblings than otherwise.

He explained that, too. The average age of all the older siblings that responded and the the average age of all the younger siblings that responded were within 1 year.

The drawback of any survey driven study, right?

And what % was before and what % after ?

5,523 before December 25, when I posted the pre-registrations. 2,254 after.

Of the before-posting people, 2199 had exactly one sibling, and 71.1% of those were the older.

Of the after-posting people, 776 had exactly one sibling, and 72.3% of those were older.

A t-test revealed no difference between the two groups, p = 0.527.

it may also reflect that firstborns just have more of an interest in birth order effects so were more likely to respond