Time to Go? Duration Dependence in Forced Migration

In this paper we introduce some theoretical and methodological refinements to account for effects of duration dependence in forced migration flows that have not been noticed in previous research. The engine of previous theoretical arguments on forced migration has been a decision theoretic model in which the potential migrants estimate the threat to their security and then weigh this threat against costs and benefits of leaving. Moreover, in previous arguments and empirical tests the costs and benefits for relocating have been treated as being the same for all people in a country. We relax this assumption, allowing the costs and benefits of relocating to be different for different individuals. This implies that some people more readily relocate than others. Over time this generates a selection effect in the population that remains behind, such that the remaining population will become increasingly unwilling or unable to relocate. The implications of our theoretical refinement are borne out empirically. Contrary to previous research we find that the accumulated stock of forced migrants decrease rather than increase the probability of new migration. Furthermore, we find that forced migrant flows abate rather than soar over time.本研究针对此前研究未予关注的强迫性移民（forced migration）流中的持续时间依赖效应，提出若干理论与方法论层面的改进方案。此前关于强迫性移民的理论论证，核心依托决策理论模型：潜在移民先评估自身安全面临的威胁，再将该威胁与离开的成本与收益进行权衡。此外，此前的论证与实证检验均假设，一国境内所有个体的迁移成本与收益完全一致。本研究放宽了这一假设，允许不同个体的迁移成本与收益存在差异。这意味着部分个体相较于他人更易产生迁移意愿。随着时间推移，这一机制会在留守人口群体中形成选择效应，使得留守群体愈发不愿或无力进行迁移。我们的理论改进所推导出的结论，得到了实证结果的验证。与此前研究结论相悖，我们发现：强迫性移民的累计存量会降低而非提升新移民发生的概率。此外，我们还发现，强迫性移民流体会随时间推移逐渐衰减，而非激增。