particle physics - Similar masses and lifetimes of the $Delta$ baryons

Why do the four spin 3/2 $\Delta$ baryons have nearly identical masses and lifetimes despite their very different $u$ and $d$ quark compositions?

Answer

Because of $u\leftrightarrow d$ isospin symmetry. For a more detailed explanation, see e.g. chapter 8 of 't Hooft's lecture notes. The pdf file is available here.