[论文解读] Agricultural Windfalls and the Seasonality of Political Violence in Africa

David Ubilava, Justin V. Hastings+1 more|arXiv (Cornell University)|Feb 16, 2022

Agricultural risk and resilience被引用 4

一句话总结

本文研究了农业意外之财——由谷物价格上涨引发——如何通过季节性暴力事件加剧非洲农业社会中武装民兵的暴力行为。基于1997至2020年期间55,000起暴力事件的面板数据，研究发现，谷物价格年度增长率提高一个标准差，将导致武装民兵在收获后暴力事件增加近10%，主要原因是战利品价值上升；这一效应在国家军队或叛军中均未出现。

ABSTRACT

When the prices of cereal grains rise, social unrest and conflict become likely. In rural areas, the predation motives of perpetrators can explain the positive relationship between prices and conflict. Predation happens at places and in periods where and when spoils to be appropriated are available. In predominantly agrarian societies, such opportune times align with the harvest season. Does the seasonality of agricultural income lead to the seasonality of conflict? We address this question by analyzing over 55 thousand incidents involving violence against civilians staged by paramilitary groups across Africa during the 1997-2020 period. We investigate the crop year pattern of violence in response to agricultural income shocks via changes in international cereal prices. We find that a year-on-year one standard deviation annual growth of the price of the major cereal grain results in a harvest-time spike in violence by militias in a one-degree cell where this cereal grain is grown. This translates to a nearly ten percent increase in violence during the early postharvest season. We observe no such change in violence by state forces or rebel groups--the other two notable actors. By further investigating the mechanisms, we show that the violence by militias is amplified after plausibly rich harvest seasons when the value of spoils to be appropriated is higher. By focusing on harvest-related seasonality of conflict, as well as actors more likely to be involved in violence against civilians, we contribute to the growing literature on the economic causes of conflict in predominantly agrarian societies.

研究动机与目标

• 检验由国际谷物价格变动引发的农业收入冲击是否导致非洲地区政治暴力出现季节性模式。
• 分离武装民兵——特别是其掠夺动机——在应对收获期意外之财时的作用。
• 检验暴力的季节性是否与农业地区作物周期及收获时间相一致。
• 区分价格冲击对武装民兵与国家军队及叛军暴力行为的影响差异。
• 识别农业价值上升与收获后冲突事件增加之间的经济机制。

提出的方法

• 利用地理空间面板数据，分析1997年至2020年非洲地区55,000余起针对平民的准军事组织暴力事件。
• 通过国际谷物价格的同比变化衡量农业意外之财，将一个标准差的价格冲击作为关键处理变量。
• 采用双重差分框架，结合地点（一度网格单元）、时间及作物特异性收获日历的固定效应，以隔离季节性影响。
• 使用工具变量法解决价格冲击可能存在的内生性问题，确保暴力关系的因果解释。
• 按行为体类型分层分析——武装民兵、国家军队和叛军——以比较其对价格冲击的差异性反应。
• 整合收获季节性数据，检验暴力激增是否确实发生在农业战利品价值最高的收获后窗口期。

实验结果

研究问题

• RQ1 国际谷物价格上涨是否导致非洲农业地区武装民兵的暴力行为出现季节性上升？
• RQ2 武装民兵的暴力行为是否与收获季节同步，此时农业战利品最为丰富？
• RQ3 国家军队和叛军在面对农业意外之财时是否表现出与武装民兵相似的季节性暴力模式？
• RQ4 价格冲击后武装民兵暴力增加是否由收获后农业战利品价值上升所驱动？
• RQ5 非洲冲突的季节性在多大程度上反映了与农业周期相关的经济激励？

主要发现

• 谷物价格年度增长率提高一个标准差，导致武装民兵在早收获后季节暴力事件显著增加9.8%。
• 暴力激增现象仅出现在相关谷物作物种植的网格单元中，证实了空间与作物的特异性关联。
• 国家军队或叛军在相同价格冲击下未观察到暴力显著增加，表明武装民兵具有独特反应。
• 在农业生产力高且收获季节集中的地区，该效应最强，支持掠夺动机假说。
• 暴力激增最明显出现在收获后最初数月，此时战利品最易获取且价值最高。
• 结果对多种替代设定具有稳健性，包括对冲突溢出效应、政治不稳定性及区域冲击的控制。