Age and the Nature of Innovation
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Are there some kinds of discoveries that are easier to make when young, and some that are easier to make when older?
Obviously yes. At a minimum, innovations that take a very long time basically have to be done by older innovators. So what kinds of innovations might take a long time to complete? Perhaps those that draw on deep wells of specialized knowledge that take a long time to accumulate. Or perhaps those that require grinding away at a question for years and decades, obsessively seeking the answers to riddles invisible to outsiders.
What about innovations that are easier when young? Well, we can at least say they shouldn’t be the kinds of innovations that take a long time to achieve. That means discoveries that can be made with years, not decades, of study. But what kinds of innovations that don’t take long study to make are still sitting around, like unclaimed $20 bills on the sidewalk?
One obvious kind of unclaimed innovation is the kind that relies on ideas that have only been very recently discovered. If people learn about very new ideas during their initial training (for example, for a PhD), then we might expect young scientists to disproproportionately make discoveries relying on frontier knowledge. At the same time, we might look for signs that older scientists build on older ideas, but perhaps from a place of deeper expertise. Indeed, we have some evidence this is the case.
Age, Frontier Ideas, and Deepening Expertise
Let’s start with Yu et al. (2022), a study of about 7mn biomedical research articles published between 1980 and 2009. Yu and coauthors do not know the age of the scientists who write these articles, but as a proxy they look at the time elapsed since their first publication.
Below are several figures, drawn from data in their paper, on what goes into an academic paper at various stages of a research career. In the left column, we have two measures drawn from the text of paper titles and abstracts. Each of these identifies the “concepts” used in a paper’s title/abstract: these are defined to be the one, two-, and three-word strings of text that lie between punctuation and non-informative words. The right columns relies on data from the citations made by an article. In each case, Yu and coauthors separately estimate the impact of the age of the first and last author.1 Moreover, these are the effects that remain after controlling for various other factors, including what a particular scientist does on average (in economics jargon, they include author fixed effects). Together, they generally tell a story of age being associated with an increasing reliance on a narrower set of older ideas.
Source: Regression coefficients with author fixed effects in Tables 4 and 5 of Yu et al. (2022)
Let’s start in the top left corner - this is the number of concepts that appear in a title or abstract which are both younger than five years and go on to be frequently used in other papers. Measured this way, early career scientists are more likely to use recent and important new ideas. Moving to the top-right figure, we can instead look at the diversity of cited references. We might expect this to rise over a career, as scientists build a larger and larger knowledge base. But in fact, the trend is the opposite for first authors, and mixed at best for last authors. At best, the tendency to expand the disciplinary breadth of references as we accumulate more knowledge is offset by rising disciplinary specialization.
Turning to the bottom row, on the left we have the average age of the concepts used in a title and abstract (here “age” is the number of years that have elapsed since the concepts were first mentioned in any paper), and on the right the average age of the cited references (that is, the number of years that have elapsed since the citation was published). All measures march up and to the right, indicating a reliance on older ideas as scientists age.
This is not a phenomenon peculiar to the life sciences. Cui, Wu, and Evans (2022) compute some similar metrics for a wider range of fields than Yu and coauthors, focusing their attention on scientists with successful careers lasting at least twenty years and once again proxying scientist age by the time elapsed since their first paper was published. On the right, we again have the average age of cited references; these also rise alongside scientist age.
Source: Regression coefficients with author fixed effects in Tables 4 and 5 of Yu et al. (2022)
On the left, we have a measure based on the keywords the Microsoft Academic Graph assigns to papers (of which there are more than 50,000). Between two subsequent years, Cui and coauthors calculate the share of keywords assigned to a scientist’s papers which recur in the next year. As scientists age, their papers increasingly get assigned the same keywords from year to year (though note the overall effect size is pretty small), suggesting deeper engagement with a consistent set of ideas.
Lastly, we can look outside of science to invention. Kalyani (2022) processes the text of patents to identify technical terminology and then looks for patents that have a larger than usual share of technical phrases (think “machine learning” or “neural network”) that are not previously mentioned in patents filed in the preceding five years. When a patent has twice as many of these new technical phrases as the average for its technology type, he calls it a creative patent. He goes on to show these “creative” patents are much more correlated with various metrics of genuine innovation (see the patent section of Innovation (mostly) gets harder for more discussion).
Kalyani does not have data on the age of inventors, but he does show that repeat inventors produce increasingly less creative patents as time goes by.
From Kalyani (2022)
This figure shows, on average, an inventor’s first patent has about 25% more new technical phrases than average, their second has only 5% more, and the third patent has about the same number of new technical phrases as average. Subsequent patents fall below average. This is consistent with a story where older inventors increasingly rely on older ideas.
As discussed in more detail in the post Age and the Impact of Innovations, over the first 20 years of a scientists career, the impact of a scientist’s best work is pretty stable: citations to the top cited paper published over some multi-year timeframe is pretty consistent. The above suggests that might conceal some changes happening under the hood though. At the outset, perhaps a scientist’s work derives its impact through engagement with the cutting edge. Later, scientists narrow their focus and impact arises from deeper expertise in a more tightly defined domain.
Conceptual and Experimental Innovation
So far we’ve seen some evidence that scientific discoveries and inventions are more likely to draw on recent ideas when the innovator is young, and an older, narrower set of ideas (plus deeper expertise?) when the innovator is older. I suspect that’s because young scientists hack their way to the knowledge frontier during their training period. As scientists begin active research in earnest, they certainly invest in keeping up with the research frontier, but it’s hard to do this as well as someone who is in full-on training mode. Over a 20-40 year career, the average age of concepts used and cited goes up by a lot less than 20-40 years; but it does go up (actually, it’s pretty amazing the average age of concepts used only goes up 2 years in Yu et al. 2022).
I argued at the outset we might expect this. The young cannot be expected to make discoveries that require a very long time to bring about. But among the set of ideas that don’t take a long time to bring about, they need to focus on innovations that have not already been discovered. One way to do that is to draw on the newest ideas. But this might not be the only way.
The economist David Galenson has long studied innovation in the arts, and argues it is useful to think of innovative art as emerging primarily from two approaches. The first approach is "experimental." This is an iterative feedback driven process with only vaguely defined goals. You try something, almost at random, you stand back and evaluate, and then you try again. The second approach is “conceptual.” It entails a carefully planned approach that seeks to communicate or embody a specific preconceived idea. Then the project is executed and emerges more or less in its completed form.
Both require a mastery of the existing craft, but the experimental approach takes a lot longer. Essentially, it relies on evolutionary processes (with artificial rather than natural selection). It's advantage is that it can take us places we can't envision in advance. But, since it takes so long to walk the wandering path to novelty, Galenson argues that in the arts, experimental innovators tend to be old masters.
The Bathers, by Paul Cezanne, one of Galenson’s experimental innovators. Begun when Cezanne was 59.
Conceptual approaches can, in principle, be achieved at any point in a lifecycle, but Galenson argues there are forces that ossify our thinking and make conceptual innovation harder to pull off at old ages. For one, making a conceptual jump seems to require trusting into a radically simplified schema (complicated schema are too hard to plan out in advance) from which you can extrapolate into the unknown. But as time goes on, we add detail and temper our initial simplifications, adding caveats, carveouts and extensions. We no longer trust the simple models to leap into the unknown. Perhaps for these reasons, conceptual innovators tend...