diff --git a/lib/rails/observers/active_model/observing.rb b/lib/rails/observers/active_model/observing.rb
index 3f01a90..8206b4b 100644
--- a/lib/rails/observers/active_model/observing.rb
+++ b/lib/rails/observers/active_model/observing.rb
@@ -338,7 +338,10 @@ def observed_class
     # Start observing the declared classes and their subclasses.
     # Called automatically by the instance method.
     def initialize #:nodoc:
-      observed_classes.each { |klass| add_observer!(klass) }
+      klasses = observed_classes
+      klasses = (klasses + klasses.map(&:descendants).flatten).uniq
+      self.class.observe(klasses)
+      klasses.each { |klass| add_observer!(klass) }
     end
 
     def observed_classes #:nodoc:
diff --git a/lib/rails/observers/activerecord/observer.rb b/lib/rails/observers/activerecord/observer.rb
index 5218008..760c87a 100644
--- a/lib/rails/observers/activerecord/observer.rb
+++ b/lib/rails/observers/activerecord/observer.rb
@@ -96,11 +96,6 @@ class Observer < ActiveModel::Observer
 
     protected
 
-      def observed_classes
-        klasses = super
-        klasses + klasses.map { |klass| klass.descendants }.flatten
-      end
-
       def add_observer!(klass)
         super
         define_callbacks klass
diff --git a/rails-observers.gemspec b/rails-observers.gemspec
index 52526ad..415a562 100644
--- a/rails-observers.gemspec
+++ b/rails-observers.gemspec
@@ -18,7 +18,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'activemodel', '>= 4.0'
 
-  s.add_development_dependency 'minitest',     '>= 3'
+  s.add_development_dependency 'minitest',     '>= 3', '< 5'
   s.add_development_dependency 'railties', '>= 4.0'
   s.add_development_dependency 'activerecord', '>= 4.0'
   s.add_development_dependency 'actionmailer', '>= 4.0'
diff --git a/test/lifecycle_test.rb b/test/lifecycle_test.rb
index fb55206..ca5a3b4 100644
--- a/test/lifecycle_test.rb
+++ b/test/lifecycle_test.rb
@@ -131,7 +131,7 @@ def test_before_destroy
   def test_auto_observer
     topic_observer = TopicaAuditor.instance
     assert_nil TopicaAuditor.observed_class
-    assert_equal [Topic], TopicaAuditor.observed_classes.to_a
+    assert_equal [Topic, Reply, AroundTopic], TopicaAuditor.observed_classes.to_a
 
     topic = Topic.find(1)
     assert_equal topic.title, topic_observer.topic.title
@@ -161,9 +161,15 @@ def test_observing_subclasses
     developer = SpecialDeveloper.find(1)
     assert_equal developer.name, multi_observer.record.name
 
+    # SpecialDeveloper was observed when DeveloperObserver initialized
+    assert_equal [Developer, SpecialDeveloper], DeveloperObserver.observed_classes
+
     klass = Class.new(Developer)
     assert_equal klass, multi_observer.last_inherited
 
+    # Subclassing Developer adds new subclass to observed_classes
+    assert_equal [Developer, SpecialDeveloper, klass], DeveloperObserver.observed_classes
+
     developer = klass.find(1)
     assert_equal developer.name, multi_observer.record.name
   end
diff --git a/test/observing_test.rb b/test/observing_test.rb
index 120d531..c7c0304 100644
--- a/test/observing_test.rb
+++ b/test/observing_test.rb
@@ -100,7 +100,7 @@ def teardown
   test "passes observers to subclasses" do
     FooObserver.instance
     bar = Class.new(Foo)
-    assert_equal Foo.observers_count, bar.observers_count
+    assert_equal [Foo, bar], FooObserver.observed_classes
   end
 end
 
@@ -124,26 +124,26 @@ def teardown
   end
 
   test "tracks implicit observable models" do
-    instance = FooObserver.new
-    assert_equal [Foo], instance.observed_classes
+    FooObserver.instance
+    assert_equal [Foo], FooObserver.observed_classes
   end
 
   test "tracks explicit observed model class" do
     FooObserver.observe ObservedModel
-    instance = FooObserver.new
-    assert_equal [ObservedModel], instance.observed_classes
+    FooObserver.instance
+    assert_equal [ObservedModel], FooObserver.observed_classes
   end
 
   test "tracks explicit observed model as string" do
     FooObserver.observe 'observed_model'
-    instance = FooObserver.new
-    assert_equal [ObservedModel], instance.observed_classes
+    FooObserver.instance
+    assert_equal [ObservedModel], FooObserver.observed_classes
   end
 
   test "tracks explicit observed model as symbol" do
     FooObserver.observe :observed_model
-    instance = FooObserver.new
-    assert_equal [ObservedModel], instance.observed_classes
+    FooObserver.instance
+    assert_equal [ObservedModel], FooObserver.observed_classes
   end
 
   test "calls existing observer event" do