Baby boom or what?

Here you see the birth distribution according to year and month for voters in the states of Miranda, Carabobo and Delta Amacuro. The "October Hump" is incredibly similar for every state I have checked out.

Mind: Miranda and Carabobo are densely populated, central states. Average voter's age is 44. Delta Amacuro is one of the most thinly populated states (after BolĂ­var and Amazonas) and one with a high proportion of native Americans living in very poor conditions (the Waraos). Its average voter's age is 41.

How is it possible that there is a child boom in October only for the 1966-1976 period (one year less or more)? Even if you always have trends - one month more births than in others -, the distribution should be more widespread through months and years. Journalist Stephen Bodzin had this hypothesis that parents could be reporting in October in order to give an advantage to their children, who would then be go to a class where most are younger. I somehow don't buy this, it is just too general, the pattern seems the same in every municipality I have seen and I don't think millions of parents in Venezuela were thinking like this. Do you have any idea?Do you think Stephen may be right?

Delta Amacuro