The Maximized Inner Rectangle Approach (MIRA) -- Disentangling Rectangularization

Rectangularization - the development of an increasingly rectangular
shape of the survival curve - has been theoretically predicted and
empirically noted among humans in nearly all low mortality countries in
the last decade. Explanations that account for rectangularization and
test their applicability to empirical data, however, remain limited. We
propose a new approach that distinguishes between two different kinds of
rectangularization, inner and outer rectangularization. Inner
rectangularization refers to the rectangular shape produced by the
mortality schedule, whereas outer refers to the used maximum living
potential determined by the highest attained age. This allows us to
implement the Maximized Inner Rectangle Approach (MIRA), which
determines the biggest rectangle under the survival curve, thereby
enabling a decomposition of the area below and above the survival curve.
The empirical application of our models reveals that rectangular
movement is not a new development, and has been a visible feature of the
survival curve since the earliest survival data we have access to.
Furthermore, our approach provides empirical evidence against
compression as a mechanism underlying rectangularization, and supports
the explanation that rectangularization is the result of a premature
mortality decline.