diff --git a/cmake/GtsamBuildTypes.cmake b/cmake/GtsamBuildTypes.cmake
index 9058807adc..e63fbf1dd4 100644
--- a/cmake/GtsamBuildTypes.cmake
+++ b/cmake/GtsamBuildTypes.cmake
@@ -190,7 +190,7 @@ if(${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
 endif()
 
 if (NOT MSVC)
-  option(GTSAM_BUILD_WITH_MARCH_NATIVE  "Enable/Disable building with all instructions supported by native architecture (binary may not be portable!)" ON)
+  option(GTSAM_BUILD_WITH_MARCH_NATIVE  "Enable/Disable building with all instructions supported by native architecture (binary may not be portable!)" OFF)
   if(GTSAM_BUILD_WITH_MARCH_NATIVE AND (APPLE AND NOT CMAKE_SYSTEM_PROCESSOR STREQUAL "arm64"))
     # Add as public flag so all dependant projects also use it, as required
     # by Eigen to avid crashes due to SIMD vectorization:
diff --git a/cmake/HandleEigen.cmake b/cmake/HandleEigen.cmake
index b21d16885f..c49eb4f8e1 100644
--- a/cmake/HandleEigen.cmake
+++ b/cmake/HandleEigen.cmake
@@ -1,7 +1,10 @@
 ###############################################################################
 # Option for using system Eigen or GTSAM-bundled Eigen
-
-option(GTSAM_USE_SYSTEM_EIGEN "Find and use system-installed Eigen. If 'off', use the one bundled with GTSAM" OFF)
+# Default: Use system's Eigen if found automatically:
+find_package(Eigen3 QUIET)
+set(USE_SYSTEM_EIGEN_INITIAL_VALUE ${Eigen3_FOUND})
+option(GTSAM_USE_SYSTEM_EIGEN "Find and use system-installed Eigen. If 'off', use the one bundled with GTSAM" ${USE_SYSTEM_EIGEN_INITIAL_VALUE})
+unset(USE_SYSTEM_EIGEN_INITIAL_VALUE)
 
 if(NOT GTSAM_USE_SYSTEM_EIGEN)
   # This option only makes sense if using the embedded copy of Eigen, it is
@@ -11,7 +14,7 @@ endif()
 
 # Switch for using system Eigen or GTSAM-bundled Eigen
 if(GTSAM_USE_SYSTEM_EIGEN)
-    find_package(Eigen3 REQUIRED)
+    find_package(Eigen3 REQUIRED) # need to find again as REQUIRED
 
     # Use generic Eigen include paths e.g. <Eigen/Core>
     set(GTSAM_EIGEN_INCLUDE_FOR_INSTALL "${EIGEN3_INCLUDE_DIR}")