Tuesday 23 June 2020

More on comparative advantage and the gender gap in STEM

Back in 2018, I wrote a post on comparative advantage and the gender gap in STEM, based on two research papers, where I noted:
So, even though female students may be better than male students in STEM subjects at school, we may see fewer of them studying those subjects at university (let alone taking them as a career), because female students are also better in non-STEM subjects at school, and they are better by more in non-STEM than in STEM, compared with male students. Economists refer to this as the students following their comparative advantage. Female students have a comparative advantage in non-STEM, and male students have a comparative advantage in STEM subjects.
In this post, I want to build on that by summarising two other research papers. The first is this article by Thomas Breda (Paris School of Economics) and Clotilde Napp (Paris-Jourdan Sciences-Economiques), published in the journal Proceedings of the National Academy of Sciences in 2018 (open access). Breda and Napp used data from the 2012 wave of PISA, covering some 300,000 15-year-old students across 64 countries. They showed that, in the PISA data:
...boys outperform girls in math by about 10% of a SD... In contrast, girls outperform boys by about a third of a SD in reading. Together, these observations suggest that girls have a comparative advantage in reading, something that appears more strikingly when we look at the gender gap in the difference between math and reading (MR) ability....
Breda and Napp then construct a measure of students' intentions to pursue maths-intensive studies and careers. They found that:
The gender gap in intentions cannot be explained by differences in math ability across genders...
That makes a lot of sense, because simply being good at maths isn't enough to encourage students to follow through on math. That depends on their comparative advantage - that is, is the student good at maths but better at other disciplines? When looking at the relationship between intensions and the difference between maths and reading (MR), Breda and Napp found that:
...the gender gap in intentions to pursue math-intensive studies and careers disappears almost entirely when one controls for individual-level differences in ability between math and reading.
In other words, the intention to study maths is more associated with the difference between maths and reading ability than it is by maths ability alone. On top of that, the difference between maths and reading ability does a better job of explain intentions than self-perceived maths ability.

The second paper (still a working paper), by Sofoklis Goulas (Stanford University), Silvia Griselda (University of Melbourne), and Rigissa Megalokonomou (University of Queensland), takes the concept of comparative advantage one step further. Their concept of comparative advantage is not just the difference between a high school student's average performance in STEM and non-STEM subjects, compared with the differences for other students in their class. What Breda and Napp refer to as comparative advantage, Goulas et al. refer to as absolute advantage. I think I prefer the Goulas et al. conception, because it more clearly conforms to what we think of as comparative advantage in a trade context - comparing opportunity costs of production between countries is analogous to comparing relative performance in STEM/non-STEM between students. A within-student comparison (like Breda and Napp) is more like a within-country comparison of production costs, i.e. absolute advantage.

Anyway, Goulas et al. have data from over 70,000 Grade 10 Greek students from 123 high schools over the period from 2001 to 2009. One of the interesting aspects of their data is that these students are assigned to classes automatically based on their surname (alphabetically). This means that they are essentially randomly allocated to classroom peers, which is important in overcoming selection bias (as I noted in Sunday's post on peer effects). Their measure of comparative advantage was the in-class ranking for each student, in terms of the difference in their average grades between STEM (algebra, physics, and chemistry) and non-STEM (modern Greek, Greek literature, and ancient Greek). Class ranking is a measure of relative performance in the class, for a group of students that it would be natural for students to compare themselves to (and for whom they probably have good information about).

Using this measure, Goulas et al. found that:
Females perform, on average, significantly higher than males in almost every subject... females' over-performance are even higher in non-STEM (=1.594) compared to STEM (=0.349)... Combining these, females have a lower comparative advantage in STEM subjects compared to males (0.409 for females and 0.487 for males).
Making use of their measures of absolute advantage (difference in average grades) and comparative advantage (within-class rank), they then look at the effects on future applications to STEM programmes in Grade 11. Focusing on the comparative advantage results, they found that:
The estimated coefficient of comparative STEM advantage is not significant for males but it is significant and equal to 0.19 for females (=0.030+0.161). This means that females who are ranked at the top of their classroom distribution in grade 10, are roughly 19% more likely to enroll in a STEM track in grade 11 than females who are ranked at the bottom of their classroom distribution, ceteris paribus...
Our findings suggest that between 4 and 6 percentage points of the 34-percentage-point gender gap (or 12-18%) in initial STEM specialization in high school are attributable to the influence of the comparative STEM advantage.
Looking at longer-term outcomes, Goulas et al. also find that comparative advantage in STEM in Grade 10 leads to a higher probability of applying to a degree-level STEM programme in university. The difference between being at the top and being at the bottom of the classroom distribution leads to a 10 percent higher likelihood of applying to a STEM degree programme. These results all appear to hold when comparing only with classmates of the same gender, when comparing at the school level rather than the class level, when changing the definition of what counts as STEM or non-STEM, and in a number of other robustness checks.

So, why does comparative advantage have such a large effect for female students, but not male students? Goulas et al. pose two mechanisms. First, they suggest lower monetary returns for women in STEM-related fields, which reduces the returns to STEM-related study. However, this is hard to reconcile with the Breda and Napp paper, which notes that the gender wage gap in STEM-related occupations is lower than for non-STEM-related occupations. The second mechanism is different preferences for STEM occupations. STEM occupations tend to be more competitive, and there is a gender gap in competitiveness (see this 2014 post, for example). Societal and environmental influences (including parents), and a lack of role models (which has been suggested as an important factor in female students not studying economics) could also contribute to this.

Coming back to the Breda and Napp article, they have an interesting suggestion on how to close the gender gap in enrolments, given the high contribution of comparative advantage:
As the gender gap in reading performance is much larger than that in math performance, policymakers may want to focus primarily on the reduction of the former. Systematic tutoring for low reading achievers, who are predominantly males, would be a way, for example, to improve boys’ performance in reading.
Redirecting education resources towards boys in order to reduce the gender gap in STEM would no doubt strike many people as counter-intuitive. It also comes with ethical issues. If STEM-related occupations are higher paying, then redirecting (male) students so that they instead study non-STEM-related subjects doesn't necessarily strike me as morally unambiguous solution. Breda and Napp make some noises in that direction but avoid being explicit about the ethical problems, while Goulas et al. more-or-less ignore the policy prescription and associated ethical issues. However, sooner or later, if we are serious about addressing the gender gap, we will have to engage with the ethical implications.

[HT: Marginal Revolution for the Breda and Napp paper; The Conversation for the Goulas et al. paper]

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