This fascinating paper (pdf) by Seth G. Benzell and Kevin Cooke argues that intermarriage among European royal houses played a significant role in decreases in the number of wars in early modern Europe:
Specifically, we show exogenous increases in kinship network distance lead to an increased likelihood of war. While our framework predicts we should also see a decreased likelihood of fighting as allies, evidence on this is inconclusive. We conclude that the rise in kinship connections is an important factor in the well-known long-run decrease in the frequency of war.
We tentatively estimate that increased kinship ties explain 30 percent of the decrease in European warfare. Suppose, conservatively, that the presence of a kinship connection reduces the chance of war between a pair of states by .67 percent. This number is a lower bound on the effect of disconnecting onpath deaths in the latter part of our sample. Relative to pre-1600, the share of monarchs connected after 1800 increased by 53 percentage points. Preventing all these new ties from being formed would therefore be expected to increase the share of dyads at war after 1800 from 1.17% to 1.53%. This is approximately 30 percent of the decrease in war over the same period.
Our data provides a rich environment to study the influence of interpersonal relationships on long-run macroeconomic, political and institutional outcomes. In this paper, we have focused on the relationship between kinship and conflict. However, the same data and network tools might well be applied to more traditional economic questions. We think future work investigating the long-run implications of kinship networks for trade, growth, and development will be fruitful. Another interesting path for future study is explicitly modeling the network formation in this environment. Strategic marriages played a major role in international relations during our period of interest. Building a structural model of strategic marriage and fertility decisions is also an interesting direction for future research.
We New Worlders, almost all of whom grew up in republics, always find odd to think that the mere fact that Prince X married (or refused) Princess Y would change the lives of millions of people. But it did!