Query: What does research show about the FBO effect?
Context: Known as the fraternal birth order (FBO) effect, this theory has been backed up by strong evidence of its prenatal origin, although no evidence thus far has linked it to an exact prenatal mechanism. However, research suggests that this may be of immunological origin, caused by a maternal immune reaction against a substance crucial to male fetal development during pregnancy, which becomes increasingly likely after every male gestation. As a result of this immune effect, alterations in later-born males' prenatal development have been thought to occur.