How should the defense activities of allies be aggregated to determine
the alliance-wide level of defense? Two alternative models-best shot
and weakest link-are contrasted with simple summation of defense spend
ing or manpower for aggregating allies' defense efforts. We extend the
joint product model to include these methods of aggregation, and devi
se an empirical procedure to test between best-shot and weakest -link
models. We apply this test to four alliances: Triple Alliance (1880-19
14), Triple Entente (1880-1914), Warsaw Pact (1963-1987), and NATO (19
61-1987). The testing procedure can be applied to other collective cho
ice situations.