Why are there so many birthdays in August?

The seasonality of births in India

One of my fond memories while growing up in Rawatbhata, a small town (and now a city!) of Rajasthan was birthday parties. These birthday parties used to be simple. We were all a bunch of similar-aged kids and would just invite each other for a mini celebration.

Everything about these parties was simple. The people were simple. The invitation was simple - you visited each house and verbally invited everyone. The food was simple. There were no starters, a couple of large bottles of ThumpsUp would suffice and was often served in steel glasses which would come out just for this occasion. There was only one simple course - the main course with pulao (rice + sautéed veggies), poori (fried flatbread), dessert (gulab jamun or kheer), chole (chickpeas), and well, a piece of cake. Paneer had not entered the menu yet, though it was sometimes on the menu if you were invited home for dinner.

The music was simple and the dance was simple. Bollywood’s top four dance numbers would play on a loop from a Philips cassette player that everyone had. It was available at a discount for everyone or I recall. And yes, there were simple “return gifts” - a Nataraj pencil and a Non-dust eraser. Simple times.

There were no bakeries in Rawatbhata for a long time, and almost all cakes were home-baked. No one in the town had ovens! There were no microwaves either. The only electrical gadget in the kitchen used to be a mixer grinder. How do you “bake” without an “oven”? The technique uses a simple idea - use sand to provide controlled heating. Too much digression. But the cakes were simple too.

Baking the cake on sand and stove! Source

While we would get invited to all the birthdays, I do remember sneaking in uninvited to a few, once in a while. I also distinctly remember August being a month of celebrations. Of course, there was Independence Day on the 15th lined up with five birthdays in August! Five! Most other months would have one or two. Naturally, August was also my favourite month - loads of good food and return gifts. People with birthdays in August had to be extra diligent. While the simple birthday menu rarely changed, the “return gifts” required deeper thought. You did not want to be called out for repeating a return gift in the same month.

Caught in the nostalgia of food, birthdays, and 90s music, I had a question going around in my head: Were the five birthdays in August a rare event? There are 365 days in a year and assuming no day is special, there is no reason August should be a non-simple birthday month. How do I find out if there is anything special about August, if there is? My first thought was to poll my odd group of college friends and ask them for their birthdays. However hard I try to avoid it, this would be a convenient sample, and I don’t think I would have learned anything mechanistic about the “why” or “how”.

What do I need to answer my question? My question can be asked more simply: when do most Indians celebrate their birthdays? If I had access to all the Aadhaar data, this question could be answered using probably a few lines of code. Aadhaar is of course closed for these use cases. A little bit of search landed me on a wonderful resource of HMIS. The description on the website is self-explanatory:

This portal will be a gateway to wealth of information regarding the health indicators of India. The information available on this portal is derived data from data uploaded by the States/ UTs. HMIS data is specifically designed to support planning, management, and decision making based on on Grading of facilities, various indicators at Block, District at State as well as National Level.

While the emphasis is on “health indicators”, HMIS has district and state-level data on how many births happen in private and public hospitals. Getting this data was quite an exercise and taught me several tricks for parsing excel/html. After struggling disproportionately with weirdly formatted files, I could extract all the birth data between 2008 and 2020 across states. With this data in hand, I could finally answer the simple question: was august the special birthday month?

I first looked at the distribution of births. September, October, and August have the highest number of births with approximately 1.9 million average births over 2008-2020. Since births peak in these months, there is an inherent “seasonality” attached to birthdays in India. In a hypothetical world, all the months would have roughly equal births, rather than having a range from 1.38 million births (April) to 1.98 million births (September).

Taking an average can sometimes hide a tonne of information that lies in a time-series data. If we look at the entire period between January 2008 and December 2020, the “seasonality” is easier to spot. From 2008 to 2020, the birth curve rises and dips over the year. The peaks happen around September/October, while April registers a deep dip.

While the data from HMIS is on births, we can infer the time of conception by simple arithmetic - subtracting 9 months. Of course, the seasonality remains intact. The peak of conceptions happens in December of the previous year or January of the same year.

Summarizing information at the country level is a good starting point. Overall, September is the month with the highest number of birthdays. But does that hold across all states? What about Rajasthan? I next broke it down at the state level and annotated the month with the highest number of birthdays.

From the figure above, August is the month with the highest number of birthdays in Rajasthan. But more importantly, the figure highlights the diversity that underlies India. While September, the month with the highest number of total births, is the month of peak births in 9 states, October is the peak month in 10 states. But there are also Meghalaya and Tripura which peak in January and December respectively. On the other extreme, July is the month of the least conceptions across multiple states. We can flip the births and look at the conception curve.

While my actual question was answered, and I discovered that there is heterogeneity within the country in how it celebrates birthdays, my related question of understanding why this happens “mechanistically” remained unanswered. I must admit beforehand that it is also a hard question to answer without access to a tonne of data.

Why are conceptions higher in a month? Why do they vary across states? Is it driven by the “wedding season” in the country?

I did not have access to the wedding registration data. So I asked a simple question: does temperature affect the rate of conception (and hence birth) in India. Surprisingly, getting temperature data for a city across a time span without paying anyone remains a non-trivial task. However after a bit of tussle and jumping language hoops, I was able to download the gridded temperature data from IMD, Pune.

To understand the relationship between temperature and rate of conceptions, I looked at the average temperature over 2008-2020 in a state and calculated its correlation with the number of estimated conceptions. For the autumn months (October/November), the correlation coefficient is the highest (-0.56). But even more importantly, this figure hinted towards the seasonality being associated with temperature. The rate of conceptions across seasons varies as Winter > Autumn > Summer > Monsoon.

I am sure you are thinking about heterogeneity, what does this relationship look like if we focus on each state individually?

When I broke down the association analysis for each state individually and arranged the states based on the strength of the correlation (between the relative percentage of conceptions every month and the mean temperature in each state), a beautiful pattern emerged. 24 out of 28 states for which I had data show a correlation coefficient of -0.5 or lower (that is, the absolute strength of correlation exceeds 0.5). For states like Manipur, Bihar, and Haryana the association between temperature and rate of conception is as high as 0.91, implying a higher temperature is associated with fewer conceptions and a one-degree drop in temperature will lead to an increase of 0.91% in conception assuming this relationship is indeed causal. States like Jammu and Kashmir and Uttarakhand which are usually colder have weak associations. While Kerala, which has a tropical climate, has a stronger association with a correlation coefficient of -0.82. Thus, the association is not as strong for colder climates, an observation that I previously made at the season level in my previous plot.

Causality is hard to prove here. We have strong associations that reproduce across states, Occam’s razor, and the golden fact that correlation does not imply causation. I wish the answer was as simple as those birthday parties.

Summer has come and passed  
The innocent can never last  
Wake me up when September ends

                - Billie Armstrong