diff --git a/actionpack/lib/action_controller/base/conditional_get.rb b/actionpack/lib/action_controller/base/conditional_get.rb
index d287ec4..4804913 100644
--- a/actionpack/lib/action_controller/base/conditional_get.rb
+++ b/actionpack/lib/action_controller/base/conditional_get.rb
@@ -102,7 +102,11 @@ module ActionController
     # Examples:
     #   expires_in 20.minutes
     #   expires_in 3.hours, :public => true
+    #   expires_in 3.hours, :private => false # this will issue public instruction
     #   expires in 3.hours, 'max-stale' => 5.hours, :public => true
+    # 
+    # Note: do not to specify contradictory public/private pairs, e.g.:
+    #   expires_in 1.hour, :public => false, :private => false
     #
     # This method will overwrite an existing Cache-Control header.
     # See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html for more possibilities.
@@ -111,7 +115,12 @@ module ActionController
 
       cache_control << "max-age=#{seconds}"
       cache_control.delete("no-cache")
-      if options[:public]
+      # To support initial 2.x API of allowing ":private => false" as per #2940
+      # the logic _should_ be to check for options[:private] == false 
+      # rather than using the implicit, which would cause incorrect behavior:
+      #   !options[:private]
+      # if-evaluation
+      if options[:public] || (options[:private] == false)
         cache_control.delete("private")
         cache_control << "public"
       else
diff --git a/actionpack/test/controller/render_test.rb b/actionpack/test/controller/render_test.rb
index acb0c89..2c5c1e7 100644
--- a/actionpack/test/controller/render_test.rb
+++ b/actionpack/test/controller/render_test.rb
@@ -61,6 +61,11 @@ class TestController < ActionController::Base
     expires_in 1.minute, :public => true
     render :action => 'hello_world'
   end
+
+  def conditional_hello_with_expires_in_with_private_false
+    expires_in 1.minute, :private => false
+    render :action => 'hello_world'
+  end
   
   def conditional_hello_with_expires_in_with_public_with_more_keys
     expires_in 1.minute, :public => true, 'max-stale' => 5.hours
@@ -1301,6 +1306,11 @@ class ExpiresInRenderTest < ActionController::TestCase
     get :conditional_hello_with_expires_in_with_public
     assert_equal "max-age=60, public", @response.headers["Cache-Control"]
   end
+
+  def test_expires_in_head_with_private_false
+    get :conditional_hello_with_expires_in_with_private_false
+    assert_equal "max-age=60, public", @response.headers["Cache-Control"]
+  end
   
   def test_expires_in_header_with_additional_headers
     get :conditional_hello_with_expires_in_with_public_with_more_keys