Who Changed on the Court

Justices are appointed for life on the assumption that we know what we are getting. Here is how far 27 of them moved, how many did not measurably move at all, and how often it was the Court that moved instead.

Data through the Supreme Court’s 2024 term, from the Supreme Court Database, 2025 Release 01. Drift is measured in percentage points of a justice’s liberal-vote share relative to the colleagues who decided the same cases.

The Short Answer

Harry Blackmun changed more than any justice of the modern era — 32.4 points, from Warren Burger’s ally to the Court’s conscience on the death penalty. Hugo Black made the same journey backwards. But the more interesting finding is how many famous “drifters” never drifted: William Brennan’s own voting moved 1.9 points in 34 terms. He stood still, and four Republican presidents rebuilt the Court to his right.

What is being measured

Counting how often a justice cast a liberal vote tells you very little on its own, because the cases change. A Court hearing mostly criminal-procedure appeals in 1965 and mostly administrative-law disputes in 1995 will produce different numbers from identical judges. So every figure on this page is a gap: the justice’s liberal-vote share minus the share for the other justices deciding the same cases. Same docket, same day, same participating colleagues.

A line above the rule means voting liberal more often than the colleagues; below, less often. Zero is not neutrality — it is agreement with the bench of the day. Drift is how far that gap moved between a justice’s first five terms and the last five.

This page measures the 1946 term onward, and only that. The database also publishes a legacy file reaching back to 1791, and 4 of the 27 justices measured here were already sitting when it ends — so for those 4, “first five terms” means their first five after 1946, not their first five on the Court. That is a deliberate limit rather than an oversight. Through 1940 the Court decided 86 percent of its cases unanimously; the Stone Court of 19411945 was the hinge, at 52 percent; and the Vinson Court that began in 1946 — the first term this page measures — was unanimous in only 31 percent. A Court that hardly ever divides leaves almost no room for any justice to sit far from the bench, and the gaps show it: their spread runs 0.6 to 3.8 points across the settled Courts, against 13.4 to 17.5 by decade in the period this page covers. A drift figure spanning that change would mostly record the Court’s change of habit and call it a change of mind. The whole-career figures are on each justice’s own profile, and the Courts themselves are compared at Every Court, Left to Right.

The gap fixes the docket problem but creates another one, and it is the reason this page is built the way it is: a gap can move because the justice moved or because the bench did. Every figure is therefore reported with both halves — how far the justice’s own share moved, and how far the colleagues’ did. Where they disagree, the page says so.

Six Justices

In every chart the darker horizontal rule is level with the bench. Above it, the justice was voting liberal more often than the colleagues deciding the same cases; below it, less often. The third figure under each chart, in bench widths, is the same move measured against how far apart the justices were at the time; it is explained in full further down. The two shaded blocks are the five-term windows the drift figure is measured between, each drawn to the height of its own margin of error. The line will not always end inside its block, and that is not an error: the line is a five-term average centred on each point and clipped at the ends, so its last point covers three terms while the block behind it covers five. Where they disagree, the block is the measured figure. See also the warning below about reading a verdict off whether the two blocks overlap.

Harry A. Blackmun§98

+32.4 ±4.2
real

The one everybody means

19691993+2020Harry A. Blackmun: -14.9 points in 1969, +21.3 in 1993
OT1969–OT1993 · +26.5 own share · 6.0 the bench · +2.03 bench widths

Richard Nixon appointed Harry Blackmun in 1970 as Warren Burger’s reliable ally. The two had been friends since childhood in St. Paul, and the press called them the Minnesota Twins. Linda Greenhouse, working from Blackmun’s own papers after his death, found that in his first five terms he voted with Burger 87.5 percent of the time in closely divided cases and with the liberal William Brennan 13 percent. By his final years those figures had very nearly traded places: Brennan 70.6 percent, Burger 32.4.

He left the Court in 1994 having written, dissenting from a denial of certiorari in Callins v. Collins, that “from this day forward, I no longer shall tinker with the machinery of death.” Both measures agree about Blackmun, which is rarer than it sounds: his own voting moved 26.5 points while the bench beneath him moved only 6.0 the other way. He is the case where the man, not the company, changed.

Hugo L. Black§76

19.7 ±4.6
real

The liberal who ended up on the other side

19461970+2020Hugo L. Black: +16.5 points in 1946, -2.6 in 1970
OT1946–OT1970 · 11.6 own share · +8.1 the bench · 1.43 bench widths

Hugo Black arrived from the Senate in 1937 and spent two decades as the Court’s great liberal. He finished it voting with the conservatives. He dissented in Griswold v. Connecticut (1965), refusing to find a right of privacy the Constitution did not name; dissented in Bell v. Maryland (1964); wrote the 5–4 majority in Adderley v. Florida (1966) upholding trespass convictions of civil-rights demonstrators, over dissents from Warren, Douglas, Brennan and Fortas; and dissented in Tinker v. Des Moines (1969) when the Court protected students wearing armbands.

Scholars generally read this as a fixed philosophy rather than a change of politics. Black’s First Amendment absolutism — “no law means no law” — covered pure speech and not symbolic conduct, so as the law expanded to protect conduct, a fixed judge produced an increasingly conservative record. Here the decomposition splits the difference honestly: his own share fell 11.6 points, and the bench moved 8.1 points the other way. Both things happened.

John Paul Stevens§101

+13.6 ±5.1
real

The Republican who kept saying he had not moved

19752009+2020John Paul Stevens: +7.2 points in 1975, +22.6 in 2009
OT1975–OT2009 · +12.0 own share · 1.7 the bench · +1.07 bench widths

Gerald Ford named John Paul Stevens in 1975, and in Regents of the University of California v. Bakke (1978) he wrote an opinion joined by Burger, Stewart and Rehnquist. He retired in 2010 as the Court’s senior liberal, having maintained throughout that his own views had held still while the Court traveled.

The data only half agrees. Stevens’s own liberal share rose 12.0 points between his first five terms and his last five, while the bench moved 1.7 points the other way. On these numbers he did move — and moved further than the Court around him.

William J. Brennan Jr.§90

+14.3 ±3.7
real

The one who did not move at all

19561989+2020William J. Brennan Jr.: +12 points in 1956, +25.7 in 1989
OT1956–OT1989 · +1.9 own share · 12.4 the bench · +1.00 bench widths

By the net figure William Brennan is the fourth-largest drifter on the Court, 14.3 points more liberal relative to his colleagues at the end than at the beginning. Almost none of it is him. His own liberal share rose 1.9 points across 34 terms — indistinguishable from unchanged. The bench moved 12.4 points to his right.

Brennan did not become a liberal lion. So far as this record can tell, he did not move at all, while Nixon, Ford, Reagan and Bush rebuilt the Court around him and the distance between them opened up. The same is true of Thurgood Marshall, whose 15.8-point drift is roughly 1.6 parts bench to one part justice. This decomposition is our own; the standard scholarship measures drift differently and classes both men as left-trending.

David H. Souter§105

+13.0 ±4.7
real

The stealth nominee

19902008+2020David H. Souter: -0.3 points in 1990, +17.6 in 2008
OT1990–OT2008 · +10.9 own share · 2.2 the bench · +0.82 bench widths

David Souter was George H. W. Bush’s 1990 nominee precisely because he had no paper trail to attack, and conservatives expected a reliable vote. He gave them one for a single term — by his second he was already voting liberal more often than his colleagues. His own liberal share then climbed 10.9 points while the bench moved 2.2 points the other way, and he retired in 2009 voting with the Court’s liberals.

Souter is sometimes compared to Blackmun. The numbers put him short of that — Blackmun moved 2.5 times as far — but in the same direction, and for a shorter tenure.

Clarence Thomas§106

0.6 ±5.4
inside the error

The immovable object

19912024+2020Clarence Thomas: -15 points in 1991, -16.5 in 2024
OT1991–OT2024 · 1.2 own share · 0.6 the bench · +0.29 bench widths

34 terms, and the gap between Clarence Thomas and the justices sitting with him is 0.6 points wider at the end than at the beginning — against a margin of error of 5.4 points. That is not a small movement so much as no measurable movement: the record cannot tell it from zero on either measure. His own share moved 1.2 points. The bench moved 0.6.

Lewis Powell is the same story and the clearest case of it — −0.3 points against 3.3, the weakest signal on the page. The second Justice Harlan looks like a third case of it and is not: his raw −1.1 fails the test too, but corrected for a bench that closed in around him it is the one figure on this page that the second measure rescues — see below. Epstein and colleagues also place Thomas among the justices showing no meaningful change across a long tenure, which is one of the points on which their method and this one agree.

Where this number lies

Three justices come out of the arithmetic pointing the wrong way — not because the sign is wrong but because a relative measure has charged the justice for something his colleagues did. For two of them the bench supplied most of the movement; for the third the bench changed shape rather than position. They are worth naming rather than burying. A different and starker version of the problem catches Tom C. Clark, Potter Stewart, Harold H. Burton and Warren E. Burger, whose own liberal shares moved in the opposite direction from their net figures; Tom C. Clark’s note below works through the sharpest of them.

Earl Warrennet 7.5

His own liberal share barely moved — down 3.1 points across 16 terms — while the bench moved 4.4 points to his left as Fortas and Marshall arrived and Goldberg left. The net figure reads as a turn to the right. What actually happened is that his colleagues caught up with him. Epstein, Martin, Quinn and Segal, using Martin-Quinn scores, class Warren as left-trending; this page’s net figure does not, and the decomposition is why.

Tom C. Clarknet 3.1

The largest personal move of any justice whose net figure points the wrong way: Clark’s own liberal share rose 14.3 points. But the Warren Court he sat on moved 17.4 points in the same direction and faster, so the gap closed and the net figure comes out negative. Read the decomposition, not the headline.

A net figure of +5.4 reads as a drift left. His own share did rise 3.3 points — but the bench he is measured against moved 2.1 points the other way over the same period, and a relative measure records that as his movement. There is a third problem underneath both: his first five terms sat on one of the widest benches measured here (18.4) and his last five on one of the narrowest (10.8), so the same distance counts for more at the end. Measured in bench widths it comes to −0.08 ± 0.40 — indistinguishable from no movement, and last of all 27 justices here. Epstein and colleagues, using a different method again, place Scalia among the justices trending right.

In each case the net figure is arithmetically correct and substantively backwards. That is a property of any relative measure, and it is why the decomposition sits beside every number here rather than in a footnote.

How much of this movement is real

A drift figure is a difference between two five-term samples, and both samples are small: a justice decides on the order of a hundred coded cases a term, so each end of the comparison rests on a few hundred votes rather than a career. Differences of a few points between two such samples can arise from which cases happened to reach the Court in those particular terms. So every figure on this page now carries an error bar, and the page reports which ones survive it.

The error comes from a case-level bootstrap: resample a justice’s cases in each window with replacement 2,000 times, recompute both his share and his colleagues’ share on the resampled cases — they move together, because they come from the same cases — and take the difference between the two windows on every draw. A drift counts as real when it exceeds twice its own standard error, the same rule this site uses to decide whether two justices can be ranked against each other at all.

Of the 27 justices with a measurable tenure, 18 clear that bar and 6 do not. The ones that do not are Samuel A. Alito Jr., Tom C. Clark, Byron R. White, John Marshall Harlan II, Clarence Thomas and Lewis F. Powell Jr. — not small drifts so much as figures the record cannot tell apart from no movement at all. The remaining 3, Felix Frankfurter, Ruth Bader Ginsburg and John G. Roberts Jr., land almost exactly on the threshold. More resampling does not rescue them: it sharpens the estimate without moving the answer, because the ratio of their drift to their own error really is about two, and which side of the line that lands on is not a fact about the justice. They are marked on the line rather than forced to one side.

The range is enormous. Blackmun’s move is 15 times its own standard error; Lewis F. Powell Jr.’s is 0.2 times, which is another way of saying he did not move.

One warning about reading this off the charts. The shaded bands mark each measured window, plus or minus twice its error. It is tempting to conclude that where the two bands overlap the justice did not really move — but that inference is wrong, and reliably so. The two windows are separate sets of cases, so the error on the difference between them is the two errors combined in quadrature rather than added — always the smaller of the two, and smaller than the eye expects. Stephen G. Breyer, Harold H. Burton, Antonin Scalia and Warren E. Burger all have visibly overlapping bands and drifts that clear the test anyway. The verdict is in the table, not in the picture.

The same moves, measured against the bench they happened on

The gap controls for which cases the Court heard. It does not control for how far apart the justices were to begin with. Five points is a long way on a Court whose members are packed within ten points of each other and an ordinary distance on one spread across nineteen — and both of those Courts are inside this dataset. Across the 54 windows measured here, the spread of the sitting justices’ gaps runs from 9.9 (Sonia Sotomayor’s first five terms, 2009–2013) to 19.4 (Lewis F. Powell Jr.’s first five terms, 1971–1975). Nearly a factor of two, between benches measured the same way.

This is a different fact from the one that keeps the pre-1946 record off this page, and worth separating carefully. That one is a collapse of the whole scale: through 1940 the Court was unanimous in 86 percent of its cases and no justice could sit far from the bench, so gaps ran a point or two rather than fifteen. Inside 1946 to 2024 there is no such trend at all — by decade the spread runs 13.4 to 17.5 points with no direction to it. What varies is which particular five terms a justice’s window happens to land on.

So each end of the comparison is also divided by the spread of the colleagues the justice was actually sitting with, which puts the move in bench widths: how far he travelled relative to the width of the Court he travelled across. Worked through on the obvious case: Blackmun began 13.9 points below a bench spread over 17.7, which is 0.79 of a bench width below it, and ended 18.6 above a bench spread over 14.9, or 1.25 above — a journey of two full bench widths.

The justice is left out of his own denominator. On a nine-member bench an outlier supplies a large share of the spread, so leaving him in would inflate the denominator for precisely the justices who sit furthest out, and shrink their own scores for no reason but their being unusual.

It changes the answer. The two orderings disagree by an average of 3.9 places, 5 justices change verdict outright, and 3 more cross onto or off the threshold:

  • Sonia Sotomayor and Elena Kagan lose their drift entirely. Both are real as raw numbers (+9.6 and +7.7), and both open on the two most tightly packed starting benches on the page — 9.9 and 11.2 against 15.5 for the median window. Measured in bench widths they fall to 23 and 22 of 27, and neither clears its error. Note what this does and does not say. Their own liberal shares did rise — the decomposition credits most of both gaps to their voting, not to the bench. What vanishes is the claim that the move was large: measured against a Court that spread out underneath them by half again, it is an ordinary distance.
  • Antonin Scalia goes further still: 17 on the raw figure, last of 27 in bench widths, and the leftward sign does not survive: +5.4 becomes −0.08 ± 0.40, which is indistinguishable from zero rather than a move to the right. His first five terms sat on one of the widest benches here and his last five on one of the narrowest. This is a third independent reason he belongs in the section above.
  • John Marshall Harlan II moves the other way — a raw −1.1 that cannot be told from nothing becomes −0.37 bench widths and rank 13, because the Court tightened around him from 16.6 to 13.3 while he held still. Standing still on a narrowing bench is itself a kind of movement.
  • Harry A. Blackmun is first on both, at +2.03 bench widths — more than 1.4 times the next largest. The headline finding does not depend on which measure you prefer.

Neither column is the right one. The raw figure answers how far the distance between a justice and his colleagues moved, in votes. The normalised figure answers whether he moved within the Court’s own range. They are different questions, and this page publishes both rather than picking. One technical note: the spread is treated as known rather than estimated, because it is not a sample — those are all the justices who sat — so the error on a normalised figure carries the measured justice’s own sampling error and nothing else.

Both orderings

All 27 justices, ranked by raw drift, with the bench-width figure beside it. Every figure carries twice its standard error, and the two verdict columns can disagree. Greyed rows are figures whose raw drift the record cannot distinguish from no movement; one of them clears the test on the second measure, which is the disagreement this table exists to show. Note also that two justices can post nearly the same drift and land on opposite verdicts: Warren E. Burger and Samuel A. Alito Jr. are three tenths of a point apart, but the first has the tighter error bar.

Ideological drift of every Supreme Court justice with at least 12 terms on the modern record, measured raw and in bench widths
#JusticeTermsDriftVerdictBench widthsVerdict#
1Harry A. Blackmun19691993+32.4 ±4.2real+2.03 ±0.26real1
2Hugo L. Black19461970−19.7 ±4.6real−1.43 ±0.32real2
3Thurgood Marshall19671990+15.8 ±3.6real+0.82 ±0.24real6(3)
4William J. Brennan Jr.19561989+14.3 ±3.7real+1.00 ±0.22real4
5William O. Douglas19461975+13.7 ±5.0real+0.66 ±0.34real7(2)
6John Paul Stevens19752009+13.6 ±5.1real+1.07 ±0.34real3(3)
7David H. Souter19902008+13.0 ±4.7real+0.82 ±0.32real5(2)
8Sonia Sotomayor20092024+9.6 ±5.6real+0.16 ±0.46inside the error23(15)
9Anthony M. Kennedy19872017+9.3 ±4.1real+0.49 ±0.30real9
10Sandra Day O'Connor19812005+8.8 ±4.3real+0.51 ±0.28real8(2)
11William H. Rehnquist19712004+8.8 ±4.1real+0.38 ±0.26real12(1)
12Elena Kagan20102024+7.7 ±5.3real+0.18 ±0.40inside the error22(10)
13Earl Warren19531968−7.5 ±3.5real−0.34 ±0.24real14(1)
14Potter Stewart19581980+6.6 ±3.8real+0.42 ±0.24real11(3)
15Stephen G. Breyer19942021+6.5 ±5.5real+0.32 ±0.34on the line15
16Harold H. Burton19461958−6.4 ±4.5real−0.26 ±0.30inside the error18(2)
17Antonin Scalia19862015+5.4 ±4.8real−0.08 ±0.40inside the error27(10)
18Felix Frankfurter19461961−4.8 ±4.7on the line−0.09 ±0.30inside the error25(7)
19Ruth Bader Ginsburg19932019+4.7 ±4.7on the line+0.45 ±0.38real10(9)
20Warren E. Burger19691985+4.4 ±3.2real+0.22 ±0.18real21(1)
21John G. Roberts Jr.20052024+4.3 ±4.8on the line+0.31 ±0.30on the line16(5)
22Samuel A. Alito Jr.20052024−4.1 ±5.1inside the error−0.26 ±0.34inside the error19(3)
23Tom C. Clark19491966−3.1 ±3.8inside the error−0.25 ±0.28inside the error20(3)
24Byron R. White19611992−2.7 ±3.7inside the error−0.11 ±0.24inside the error24
25John Marshall Harlan II19541970−1.1 ±4.5inside the error−0.37 ±0.30real13(12)
26Clarence Thomas19912024−0.6 ±5.4inside the error+0.29 ±0.38inside the error17(9)
27Lewis F. Powell Jr.19711986−0.3 ±3.3inside the error−0.08 ±0.18inside the error26(1)

Every Justice, Most Moved to Least

All 27 justices with at least 12 terms on the modern record. Every chart shares the same vertical scale, ±40 points, so they can be read against one another. “Him” is the change in the justice’s own liberal share; “bench” is the change in the colleagues’. Figures marked inside the error are drifts smaller than twice their own error — measured, and indistinguishable from no movement.

19691993Harry A. Blackmun: -14.9 points in 1969, +21.3 in 1993
him +26.5 · bench 6.0real
Hugo L. Black−19.7 ±4.6
19461970Hugo L. Black: +16.5 points in 1946, -2.6 in 1970
him 11.6 · bench +8.1real
19671990Thurgood Marshall: +8.9 points in 1967, +30.2 in 1990
him +6.1 · bench 9.7real
19561989William J. Brennan Jr.: +12 points in 1956, +25.7 in 1989
him +1.9 · bench 12.4real
19461975William O. Douglas: +16.2 points in 1946, +31.4 in 1975
him +11.0 · bench 2.7real
19752009John Paul Stevens: +7.2 points in 1975, +22.6 in 2009
him +12.0 · bench 1.7real
David H. Souter+13.0 ±4.7
19902008David H. Souter: -0.3 points in 1990, +17.6 in 2008
him +10.9 · bench 2.2real
Sonia Sotomayor+9.6 ±5.6
20092024Sonia Sotomayor: +13.1 points in 2009, +20.4 in 2024
him +6.3 · bench 3.3real
19872017Anthony M. Kennedy: -14.2 points in 1987, -1.2 in 2017
him +8.4 · bench 0.8real
19812005Sandra Day O'Connor: -13.7 points in 1981, -5.7 in 2005
him +5.1 · bench 3.8real
19712004William H. Rehnquist: -26.6 points in 1971, -15.2 in 2004
him +2.4 · bench 6.4real
Elena Kagan+7.7 ±5.3
20102024Elena Kagan: +15.1 points in 2010, +18.4 in 2024
him +4.7 · bench 3.0real
Earl Warren−7.5 ±3.5
19531968Earl Warren: +9.3 points in 1953, +7.5 in 1968
him 3.1 · bench +4.4real
Potter Stewart+6.6 ±3.8
19581980Potter Stewart: -10.6 points in 1958, -5.1 in 1980
him 9.5 · bench 16.0real
19942021Stephen G. Breyer: +9.8 points in 1994, +18.5 in 2021
him +10.2 · bench +3.7real
Harold H. Burton−6.4 ±4.5
19461958Harold H. Burton: -15.5 points in 1946, -21.4 in 1958
him +2.5 · bench +8.9real
Antonin Scalia+5.4 ±4.8
19862015Antonin Scalia: -13.9 points in 1986, -7.6 in 2015
him +3.3 · bench 2.1real
Felix Frankfurter−4.8 ±4.7
19461961Felix Frankfurter: -14.9 points in 1946, -18.8 in 1961
him 0.2 · bench +4.6on the line
19932019Ruth Bader Ginsburg: +11.6 points in 1993, +19.7 in 2019
him +9.3 · bench +4.6on the line
19691985Warren E. Burger: -21.2 points in 1969, -14.8 in 1985
him 2.3 · bench 6.7real
20052024John G. Roberts Jr.: -14.1 points in 2005, -4.5 in 2024
him +3.2 · bench 1.2on the line
20052024Samuel A. Alito Jr.: -14.9 points in 2005, -19.9 in 2024
him 5.2 · bench 1.2inside the error
Tom C. Clark−3.1 ±3.8
19491966Tom C. Clark: -2.4 points in 1949, -6.2 in 1966
him +14.3 · bench +17.4inside the error
Byron R. White−2.7 ±3.7
19611992Byron R. White: -2.1 points in 1961, -0.9 in 1992
him 19.1 · bench 16.4inside the error
19541970John Marshall Harlan II: -19.3 points in 1954, -10.9 in 1970
him 3.2 · bench 2.1inside the error
Clarence Thomas−0.6 ±5.4
19912024Clarence Thomas: -15 points in 1991, -16.5 in 2024
him 1.2 · bench 0.6inside the error
19711986Lewis F. Powell Jr.: -13 points in 1971, -10.7 in 1986
him 3.4 · bench 3.2inside the error

13 more justices appear in the database with tenures too short to measure this way — among them Goldberg, Fortas and, for now, Gorsuch, Kavanaugh, Barrett and Jackson. Five terms at each end of a career needs a career.

Method, and its limits

Built from the Supreme Court Database (2025 Release 01, justice-centered), which covers the 1946 through 2024 terms and classifies every vote as liberal, conservative, or not classifiable. One vote per justice per case. Cases the database declines to code are excluded. The database’s legacy release, which runs from 1791 to 1945, is read elsewhere on this site but deliberately not here.

Years here are October Terms, not calendar years. The Court’s term begins in October and runs into the following summer, so OT1969 covers cases decided into 1970. That is why Blackmun, sworn in during June 1970, appears from 1969 — he has eight coded cases in that first partial term against 137 in the next. Alito is the same, entering at OT2005 after a January 2006 confirmation.

The labels are the database’s, not ours. “Liberal” is a term of art defined issue by issue — pro-union in labor cases, anti-business in economic cases, pro-United-States in tax cases — and the six areas covering criminal procedure, civil rights, the First Amendment, due process, privacy and attorneys share a single combined definition. The full definitions are on the database’s codebook page.

Lines are smoothed with a centered five-term window, pooling cases rather than averaging rates. That window is clipped at the start and end of a tenure, so the first and last points on any line rest on fewer cases than the middle — Douglas’s final point on 291 cases against 774 mid-career. The vertical scale is set at a round ±40 points rather than at the observed maximum, so no single thin endpoint sets the scale the others are drawn against.

The strongest objection to all of this comes from Michael Bailey, and it bites hardest on a page about change over time. His argument is that a shift in vote-derived numbers cannot be told apart from a shift in the cases: when a justice’s conservative votes rise, it may be the justice who moved or it may be what the Court agreed to hear that year. Treating such scores as comparable across decades requires assuming the distribution of case characteristics holds still, which it does not; his own remedy is to bridge the Court to actors outside it, so that case content and ideology can be separated. Bailey, “Is Today’s Court the Most Conservative in Sixty Years?”, Journal of Politics 75 (2013): 821–34. He separately reports an objection held by some scholars, that the database’s coding can cut against political intuition: a vote to uphold campaign-spending limits is recorded as conservative because it restricts speech. The gap measure used here answers part of Bailey’s case-composition point and not all of it — comparing a justice only against colleagues deciding the same cases removes the effect of the docket at any one moment, but nothing here can rule out that the KIND of case reaching the Court changed what a liberal vote required. Readers who want the numbers taken further should start with Epstein, Martin, Quinn and Segal, Ideological Drift Among Supreme Court Justices: Who, When, and How Important?, 101 Northwestern University Law Review 1483 (2007), which finds drift to be the norm rather than the exception and reaches its conclusions with a different method (Martin-Quinn scores) that agrees with this page about Blackmun, Black and Thomas and disagrees about Warren and Scalia. Linda Greenhouse published a reply in the same review’s Colloquy.

Blackmun’s agreement figures are from Linda Greenhouse, Becoming Justice Blackmun (2005), drawn from his papers. Case citations: Griswold v. Connecticut, 381 U.S. 479 (1965); Bell v. Maryland, 378 U.S. 226 (1964); Adderley v. Florida, 385 U.S. 39 (1966); Tinker v. Des Moines, 393 U.S. 503 (1969); Regents of the University of California v. Bakke, 438 U.S. 265 (1978); Callins v. Collins, 510 U.S. 1141 (1994).

Questions

Do Supreme Court justices become more liberal over time?

More often than not, on this measure. Of the 27 justices since 1946 with a long enough record to measure, 17 ended further to the left of their colleagues than they began and 10 ended further to the right. But the pattern is weaker than the famous cases suggest: only 7 moved more than ten points, and 6 of the 27 moved by an amount that does not clear its own margin of error at all.

Which Supreme Court justice changed the most?

Harry Blackmun, by a wide margin. Appointed in 1970 as Warren Burger’s ally, he ended his tenure 32.4 points further to the left of his colleagues than he began — 1.6 times the shift of the next-largest mover, and unusual in that almost all of it was his own voting rather than a change in the bench around him.

Did Hugo Black become conservative?

On the record, yes, relative to the justices sitting with him. Black arrived in 1937 as the Court’s great liberal and finished it dissenting in Griswold and Tinker and writing the majority against civil-rights demonstrators in Adderley v. Florida. Scholars generally attribute this to a fixed judicial philosophy rather than a change of politics: his First Amendment absolutism protected pure speech but not symbolic conduct, so as the law expanded, a stationary judge produced an increasingly conservative voting record.

Do justices drift because of the job or because the Court changes around them?

Both, and this page separates them. Blackmun, Stevens, Souter and Kennedy moved mostly on their own. Brennan barely moved at all, and Marshall moved about a third as far as the bench did — in both cases the colleagues opened most of the distance, with little or no change of heart from the justice. Reporting only the net figure credits the justice for what was really a change in his colleagues. There is a third possibility the page also measures: the Court can grow more or less spread out without moving, which makes the same gap mean something different. That is what shrinks the figures for Sotomayor and Kagan when they are measured against the width of their own bench.

Is Supreme Court ideological drift statistically significant?

For most of these justices, yes, but not for all of them, and the difference is worth taking seriously. Each drift figure compares two five-term samples of a few hundred votes each, so it carries real sampling error. Measured with a case-level bootstrap, 18 of the 27 measurable drifts exceed twice their own standard error and 6 do not — Samuel A. Alito Jr., Tom C. Clark, Byron R. White, John Marshall Harlan II, Clarence Thomas and Lewis F. Powell Jr. moved by amounts the record cannot distinguish from nothing. The other 3 sit on the threshold itself. Blackmun's move is 15 times its own error and is not in any doubt.

Every justice’s full voting record, broken out by issue area, sits on their own profile. The Courts before 1946 — including the seven whose justices cannot be ranked against one another at all — are at Every Court, Left to Right.