First, left's examine recency bias in the encyclopedia. I've done this by taking David Schwitzgebel's August 2019 scrape of the bibliographic sections of all the main-page SEP entries and searching for the first occurrence of "19", "20", or "forthcoming" on each line, then retrieving the first four characters from that location. Non-numbers (except "fort") and numbers <1900 or >2019 were excluded. Everything else was interpreted as a date. (I did not include dates from before 1900, since those works citation formats are less systematic, and often a translation date is cited rather than an original publication date.)
The result is a pretty little curve peaking at 2003-2007:
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In 2014, I'd conducted a similar analysis. In those data, the peak was 1999-2003:
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And in 2010, I'd also done a similar analysis! The peak year was 2000:
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Thus, in the Stanford Encyclopedia, the most recent works appear to be somewhat disadvantaged compared to works about ten years old. Back in time from the peak years, there's a steep linear decline to about 1950, before which there are few citations and the citation rate becomes approximately flat. (Probably, serious curve fitting wouldn't show it to be three linear phases; but close enough.) Over the past nine years, the peak appears to have advanced by about five years. Since SEP entries are updated about every five years on average, we might expect some delays for that reason; and if people are a little lazy about updating references when they update their entries, that could explain why the peak isn't advancing as fast as the clock.
I assume that all these effects are recency effects. Another alternative, of course, is that early 21st century philosophy is vastly better and more citable than earlier philosophy, so that a good a 23rd century encyclopedia would show a similar curve, also massively disproportionately citing early 21st century philosophers compared to 20th century philosophers. (If you find that plausible, I have a beautiful little Proof that P to sell you!)
Based on these results, one might expect that the most-cited philosophers in the 2019 Stanford Encyclopedia would be those whose most influential works appeared around 2003-2007. However, that is not the case.
Most Cited Philosophers, Oldest Generation
For analysis, I have divided my list of the 295 most-cited philosophers into four generations based on age: 1900-1919 (oldest), 1920-1945 (pre-boom), 1946-1964 (boomers), and 1965-present (Generation X). Age was estimated based on birthyear as recorded in Wikipedia (for most authors) or estimated based on date of undergraduate degree (assuming age 22) or in a few cases date of PhD (assuming age 29). A CSV with the data is here. I welcome corrections.
Looking at the oldest generation (1900-1919), we see some stalwarts near the top of the most-cited list: Quine at #2 and Davidson at #5. Chisholm, Strawson, Popper, Geach, Goodman, Mackie, and Anscombe all appear in the top 50. Interestingly, although 19 philosophers from this generation rank among the top 100, only 13 appear in the remainder of the list of 295.
I'm inclined to attribute this to a phenomenon I call The Winnowing of Greats. This is the tendency for the difference between the top performers and the nearly-top performers in any group to come to seem larger with historical (and other types of) distance. We're still citing Quine and Davidson, and to some extent Richard Brandt (#129) and Norman Malcolm (#236), but less famous philosophers from that generation are quickly dropping off the radar.
The intuitive idea of Winnowing of Greats is this: If you're close to a field and you want to list, say, ten leaders in that field in rank order, you might list A, B, C, D, E, F, G, H, I, and J. Another person, also close, might partly agree, maybe listing A, C, B, E, G, D, I, K, L, and M. With more distance, someone might only list or think of five -- likely A, B, C (consensus top) and two of D, E, F, or G, starting to forget about H and higher. Still later, people might only mention A, B, and C. Over time, these will come to seem the consensus "best" and thus the ones who need to be discussed on grounds of historical importance in addition to whatever other reasons there are to discuss them; and others will be relatively less mentioned and mostly forgotten except by specialists, and the gap in apparent importance between the top and the remainder will grow -- eventually becoming the "consensus of history".
We could interpret such winnowing as a type of recency bias against all but the most famous, flowing from ignorance due to distance; or we could see it as a more legitimate winnowing process.
Starting somewhere around rank #50, the philosophers from the oldest generation who are still ranked might strike those of us who know the history of 20th century philosophy to be ranked rather low relative to their historical importance. I interpret this as recency bias. Quantitative evidence of recency bias is this: Looking at only those philosophers on both the 2014 and 2019 lists, the average loss in rank between the two measures was 11 spots. (Going logarithmic, the average natural log of the rank is 4.11 in 2014 and 4.29 in 2019.)
(For the curious, Chisholm was a notable decliner, rank 12 to 19, which is proportionally large in just 5 years, while Anscombe bucked the trend, climbing significantly, from 66 to 48.)
Most Cited Philosophers, Pre-Boom Generation
The dominant generation is the pre-boom generation (1920-1945). Although this generation includes the largest number of birth years, their dominance of the top of the list is too great to be explainable by that fact alone. This generation gives us six of the top ten (Lewis, Putnam, Rawls, Kripke, Williams, and Nozick) and 33 of the top 50. Most of these authors did their most influential work in the 1960s-1980s. Despite the citation curve peaking for works written in 2003-2007, foundational work by this generation is still being heavily cited. For example, the two most-cited works in the Stanford Encyclopedia are Rawls's A Theory of Justice (1971, cited in 115 entries) and Kripke's Naming and Necessity (1980, cited in 88 entries). (More data on this soon.)
Time is starting to affect the rankings of this generation, too, with an average decline in rank of 8 (average difference in ln of .05). Notably, however, in the top 50, there is an average increase in rank of 3. (It's 1.4 if we exclude Pettit, whose rank increased markedly due to a methodological change: I now include second authors.) This difference in trajectory between the top and bottom is consistent with the Winnowing of Greats.
According to a demographic theory that I call The Baby Boom Philosophy Bust, the Baby Boom generation had a substantial demographic advantage in academic philosophy in the United States. (This probably generalizes outside of the U.S. and outside of philosophy, but let me stick with what I know.) Undergraduate enrollments in the U.S. jumped from 2,444,900 in 1949-1950 to 3,639,847 in 1959-1960 to 8,004,660 to 1969-1970 to 11,569,899 in 1979-1980. After that, enrollments continued to grow, but at a much slower pace. The latter part of this period was of course when the baby boomers hit college, but the earlier part of the period was important too, in the wake of the G.I. Bill and the fast growth of the national prestige of higher education. This national prestige was, I conjecture, partly due to the prestige of the space race and the power of the atom bomb, and it extended into the humanities and arts partly due to the popularity of the idea of IQ and the emerging notion of "creativity". (I have a colleague here at UCR, Ann Goldberg, who is doing fascinating work on the history of the concept of creativity and its role in educational institutions.)
Who was hired to teach all of these new undergraduates? It was of course, the pre-boom generation. A flood of pre-boom Assistant Professors hit the universities during this period. Doing their foundational early-career work in the 1960s-1970s, they set the agenda for the philosophy of the period. Then when the boomers got their PhDs and hit the job market in the 1980s, they discovered that pre-boomers were already astride the academy -- mid-career now, at the height of their influence, not yet ready to step aside for their younger generation. The job market was terrible, and those who made it into tenure-track positions found themselves in an academic world already dominated by Rawls, Lewis, Kripke, Fodor, etc., without a lot of new space at the top. My hypothesis is that this fact about academia in the 1980s and early 1990s means that the baby boomers grew philosophically in the shade of the pre-boom generation -- and not to the heights of prestige and influence that they would have grown to, had they not been so overshadowed in their early careers.
With this hypothesis in mind....
Most Cited Philosophers, Baby-Boom Generation
The boomers (born 1946-1964) contribute two philosophers to the top ten: Nussbaum (#9) and Williamson (#10). Another five are among the top fifty: Fine, Sober, Kitcher, Hawthorne, Smith. (Hawthorne, born 1964, is right at the cutoff between Boom and Gen X, if I have his date right.) They are thus vastly underrepresented in the top 50 compared to the pre-boomers (7 vs 33). However, they are more proportionately represented in the list as a whole (113, compared to 129 for the pre-boomers).
Could the boomers rise in relative prestige, so that if we did a similar analysis in ten or twenty years, we'd find them dominating the top 50 in the way the pre-boomers do now? I see three reasons not to think so.
First, the boomers have already started declining in citation rate, comparing 2014 and 2019, with an average rank decline of 8 (ln = +.009). Mitigating this, however, if we look at the top 100, there's an overall average rank gain of 11 (ln = -.16) -- consistent with the winnowing hypothesis.
Second, in other research, I've found that philosophers tend to reach peak influence around ages 55-70. Thus, boomers should be at their peak influence now and we shouldn't expect a lot more climbing overall.
Third, as noted above, there is a strong recency bias in the Stanford Encyclopedia citations. This should tend to favor philosophers younger than the boomers, and increasingly so over time -- especially since philosophers on average tend to do their most influential work in their late 30s and 40s.
Most Cited Philosophers, Generation X
Gen Xers (born 1965-1980) are still too young to be very well represented among the top-cited philosophers in the Stanford Encyclopedia: Only 21 qualify for the list of 295, three in the top 100 (Chalmers, Schaffer, and Sider). In the past five years, the average rank gain in this group is 16 positions (ln = -.15), so, as one would expect, they are still on an upward trajectory. Also as one would expect, many of them are new to the list as of 2019 (11 of the 21), and so not included in these trajectory averages, though headed upward in another sense.
It is, I think, too early to know if Generation X will ultimately prove also to have grown too much in the shade of the pre-boom generation. I sense that this might be so: Mainstream analytic philosophers still to a large extent live in a philosophical world whose agenda was set by Lewis, Kripke, Rawls, Williams, and Putnam.
Side note on demographic diversity of most-cited Gen X philosophers: If my gender and race/ethnicity classifications are correct, then (perhaps surprisingly?) the most-cited Gen X philosophers are slightly farther from gender parity and racial diversity than the Boomers, with 3/21 women and no Latinx or non-White philosophers (compared to the Boomers' 17% women, 2% Latinx or non-White). However, since the numbers are small, this might be chance variation.
Explanation of the Misalignment of Peak Citation Year and the Age of the Most-Cited Philosophers
To cross my t's and dot my i's: Although the peak citation years are 2003-2007, the pre-boom generation is the most cited because, due to their demographic advantage in academia, they dominated philosophy from the 1970s at least into the 1990s (and maybe they still do, despite death and retirement), shading the boomers and maybe also the Gen-Xers. Although recent work is the most cited in the aggregate, the Winnowing process hasn't yet given us the distance required for consensus on the Greats, so those recent citations remain scattered among many authors.
This post is already plenty long, so I won't bother crossing my x's and dotting my j's.