  Superconductivity in cuprates peaks in the doping regime between a metal at
high p and an insulator at low p. Understanding how the material evolves from
metal to insulator is a fundamental and open question. Early studies in high
magnetic fields revealed that below some critical doping an insulator-like
upturn appears in the resistivity of cuprates at low temperature, but its
origin has remained a puzzle. Here we propose that this 'metal-to-insulator
crossover' is due to a drop in carrier density n associated with the onset of
the pseudogap phase at a critical doping p*. We use high-field resistivity
measurements on LSCO to show that the upturns are quantitatively consistent
with a drop from n=1+p above p* to n=p below p*, in agreement with high-field
Hall data in YBCO. We demonstrate how previously reported upturns in the
resistivity of LSCO, YBCO and Nd-LSCO are explained by the same universal
mechanism: a drop in carrier density by 1.0 hole per Cu atom.
