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Page Number: 53

8 

MCGIRT v. OKLAHOMA 

ROBERTS, C. J., dissenting 

[an] Act—particularly the manner in which the transaction
was negotiated with the tribes involved and the tenor of leg-
islative Reports presented to Congress.”  Ibid.  When such 
materials “unequivocally reveal a widely held, contempora-
neous  understanding  that  the  affected  reservation  would 
shrink as a result of the proposed legislation,” we will “infer 
that  Congress  shared  the  understanding  that  its  action
would diminish the reservation,” even in the face of “statu-
tory  language  that  would  otherwise  suggest  reservation 
boundaries remained unchanged.”  Ibid. 

Third,  to  a  “lesser  extent,”  we  examine  “events  that  oc-
curred after the passage of [an] Act to decipher Congress’ 
intentions.”  Ibid.  “Congress’ own treatment of the affected 
areas, particularly in the years immediately following the
opening, has some evidentiary value, as does the manner in 
which  the  Bureau  of  Indian  Affairs  and  local  judicial  au-
thorities dealt with [the areas].”  Ibid.  In addition, “we have 
recognized  that  who  actually  moved  onto  opened  reserva-
tion lands is also relevant.”  Ibid.  “Where non-Indian set-
tlers  flooded  into  the  opened  portion  of  a  reservation  and 
the area has long since lost its Indian character, we have 
acknowledged  that  de facto,  if  not  de jure,  diminishment 
may have occurred.”  Ibid.  This “subsequent demographic 
history”  provides  an  “additional  clue  as  to  what  Congress
expected would happen.”  Id., at 471–472. 

Fifteen years later, another unanimous Court described 
the  same  methodology  more  pithily  in  South  Dakota  v. 
Yankton Sioux Tribe, 522 U. S. 329 (1998).  First, the Court 
reiterated  that  the  “most  probative  evidence  of  diminish-
ment is, of course, the statutory language.”  Id., at 344 (in-
ternal quotation marks omitted).  The Court continued that 
it would also consider, second, “the historical context sur-
rounding the passage of the . . . Acts,” and third, “the sub-
sequent treatment of the area in question and the pattern 
of  settlement  there.”  Ibid.  (quoting  Hagen,  510  U. S.,  at 
411).