In late 2014, the Tumblr blog Woman Tax, created by a group going by the name of Georgette Sand, went viral in the francophone web. The French government caught up with the trend and launched an official inquiry. Its goal would be to determine whether or not women paid more for certain products, on top of earning less than men.

We got in touch with the administration in charge of the issue and learned that the report had no publication date set. It was not entirely absurd to think that the whole affair had been buried deep within the corridors of the French Ministry of Finances. We did not take someday maybe for an answer and set out to find the Woman Tax and compute how much it was. We wouldn't even stop there: We would compare European countries and see if some markets, such as egalitarian Sweden, had a lower Woman Tax than others.

The task at hand seemed easy enough. We live in Berlin and are often in Paris (Journalism++ has offices in both cities) and saw clear differences in each town. Rossman, a German drugstore, displays products for men and women side-by-side and one cannot spot a clear price difference by gender. Fly one hour west to Monoprix, a French retailer, and you will find clearly separated male and female products, the latter seemingly much more expensive.

Many articles have been written about the woman tax based on anecdotal evidence. We are datajournalists and could not be content with rumors or single case studies. We wanted to carry out a comprehensive, European survey and find the retailers and manufacturers that discriminated most against women.

Inthis study by the University of Central Florida, American researchers showed that women paid systematically more for hairdresser, dry-cleaning (shirts only) and cosmetics. Another American study showed that car dealers offered better deals to men than to women, even before any price negotiation had taken place. In several US states, the legislator even forbade – many years ago already – gender-based price discrimination in services. You won't find hairdressers charging for a man's or woman's cut but prices that depend on one's hair length. Why are France and Germany so late in this fight for gender equality? We were full of enthusiasm – we were going to measure the woman tax and find out ways to end it.

Scraping all the prices of all retailers in Europe was not much of a problem. To test our approach, we looked at deodorants at Monoprix and Rossman and at perfumes and Colognes at Douglas (a German chain) and Sephora (the leading French retailer of beauty products). These products should have been easy enough, as US researchers had already shown that cosmetics were more expensive for women.

All in all, we scraped 5000 prices for perfumes and 500 for deodorants. A little data cleaning let us compute a price per liter for each. Price averages showed that products aimed at women were much more expensive, irrespective of retailer or country. One of our starting hypotheses, that Germans, being more on the unisex side of things, would be less discriminatory, broke in pieces. One should note, however, that women sections benefit from more choices at the high-end. A 100-euro 7.5mL bottle of Shalimar (for women) makes for a €14,000/L product – that pushes the average upwards quite a bit. The project was going to be longer than planned.

We were not going to stop there. We decided next to compare products two-by-two, i.e identical products that were packaged differently for men and women. That was much harder to automate but we found enough product pairs: similar enough for the price of the ingredients to be the same (in a perfume, for instance, ingredients cost less than 5% of the final price) and gendered enough so that no doubt was possible. For deodorants at Monoprix, differences were clear but varied across brands. Rexona's deodorants were 30% more expensive for women, Nivea's were four times as expensive. Men products could also be more expensive: Cadum's products were three times more costly for men. In perfumes, the difference is more evenly spread out, women paying about 20% more.

However, we were not entirely satisfied with this methodology. What about Axe's products (for men), extremely expensive and with no female equivalent? What about unisex products? And what if price differences were due to the spatial organization of a supermarket? Should we measure the distance between the men's and the women's shelves in all of Europe's supermarkets? The project's time frame was increasing again.

One of the most well-known effects in marketing is anchorage. If you offer a client three products priced at 5, 10 and 15 euros, you will sell more 10-euro, mid-range products. Offer the same products at 5, 15 and 50 euros and you will sell almost as many mid-range products even if their price increased by 50%. Because of the anchoring effect, it does matter that some marketers offer more high-end products to women than to men. By anchoring the price of a certain product range higher, they contribute to women's spending more on cosmetics than men for similar products. Computing the woman tax was getting trickier and trickier.

The data we collected also showed that manufacturers offered more low-end products to women. Maybe the more expensive women products were simply bought by more well-off women and were sold in very small volumes. To test that hypothesis, we would have needed the sales numbers from retailers and manufacturers, something much harder that scraping prices.

The charts below show the distribution of the prices of deodorants available in Monoprix.

We felt that we were not on the right track. We called some acquaintances who worked as economists and marketers and asked for advice. For them, the issue is less about prices than about expertise. A customer who knows an area well is able to understand the differences between products and to fall for a high-end item. Someone who doesn't know anything about wine will not spend €500 for a 2003 Pommard, even though it might seem totally reasonable to an expert. Marketers at Unilever and the like know very well that women are generally more expert than men on cosmetics. They therefore extend their range of products to entice women in moving upscale and spend more.

The Woman Tax is less of a marketing problem than an educational one. Why do most women feel expert in beauty and cosmetics when most men can shamelessly know nothing in the matter and be happy and accepted with just 2 or 3 basic products in their bathroom?

The answer lies in the image that we, as a society, build around feminity. To be a women involves taking great care of one's appearance, to buy lipstick, beauty creams, to get waxed and to undergo many more procedures that add up to dozens of euros at the end of the month.

In the end, we were ready to follow this approach. The goal was not to compute the difference between gendered products but the cost of being a woman in a European society. There was still a bug: To make sense, we needed to compute the cost of being a man as well. But the cost of manhood is even more complex, because European societies tend to consider the masculine version of things to be the norm. Often, but not always. Addiction to tobacco, for instance, is all-so-expensive and very manly. It would be disingenuous to see a personal preference in this addiction, as many, if not all, European armies, distributed packs of cigarettes to conscripts. In Greece, the only remaining country with conscription, a study revealed that half of smoking men picked up the habit in the army. The same holds true for alcohol. A woman who does not drink is more socially accepted than a male teetotaler. Do the afterwork beers count as the price of manhood? And talking about expertise, rare are the women who fill up their tanks with premium gasoline, which costs 10 to 20% more than regular and whose benefits for the engine are unproven.

What seemed like an easy job evolved in a global reflexion on gender and the habits related to feminity and manhood. Interesting, but hard to measure.

Even though it was frustrating to realize that we were unable to measure a phenomenon that we felt we could see, there are still ways to address the most blatant price discrimination. For services, abuses are easy to spot. The price of a standard cut at a hairdresser and of a shirt at the dry-cleaning are much more expensive for women, without any reason. There is no need for a European, data-driven investigation on the subject. Any newsroom can do it by making 15 phone calls to hairdressers and dry-cleaners in town. We hope that lawmakers in all European countries will forbid the practice soon.

Reality is too complex for us to assert that there is indeed a Woman Tax – much less to measure it.