Investigating the "physics" of food crises consists in identifying features which are common to all large-scale food crises. One element which stands out is the fact that during a food crisis there is not only a surge in deaths but also a correlative temporary decline in conceptions and therefore in subsequent births nine months later. This scenario is studied in three cases of large-scale food crises: Finland (1868), India (1867-1907 and 1942-1944), China (1960-1961). It turns out that between the regional amplitudes of death spikes on one hand and the amplitudes of birth troughs on the other hand there is a power law relationship. This confirms the same phenomenon already observed in the wake of the epidemic of 1918 in the United States (Richmond et al. 2018b). In the second part of the paper we explain and demonstrate on three case studies how this relationship can be used for the investigation of mass-mortality episodes in cases where direct death data are either uncertain, suspicious or nonexistent.