explanatory power is trivial, however,<sup>and it is not significant when other explanatory</sup> variables, such as CEREAL, are introduced.

# **_Shocks to agriculture_**

Another important factor which could "push" people<sup>out of rural areas is a shock to</sup> agricultural production. Agricultural shocks can<sup>include not only weather shocks such as</sup> droughts - during one drought year in<sup>Mauritania, for instance, population in the capital</sup> city of Nouakchott doubled (Potts, 1995) -<sup>but also collapses in prices, disruptions in the</sup> distribution system, or unavailability of fertilizers.

Rather than trying to create separate<sup>measures of these possible disruptions, we</sup> constructed a variable that measures<sup>the difference between actual and expected crop</sup> output. POSCROPSH captures positive<sup>deviations (one or two standard deviations) from</sup> expected crop yield, while NEGCROPSH<sup>does the same for negative shocks to crop</sup> yield." We expect the sign on POSCROPSH<sup>to be positive (indicating that a better-than-</sup> expected crop yield over the previous 5 years<sup>will reduce the likelihood of a rural farmer</sup> moving to the city) and the sign on NEGCROPSH<sup>to be positive, as a bad yield would</sup> push people into the city.

Neither NEGCROPSH nor POSCROPSH<sup>are significant (regression 8). However, we</sup> did get expected results using annual shock<sup>measures (rather than the sum over the</sup> preceding 5 years). This variable is highly<sup>sensitive to sample changes, however, and the</sup> results were not always robust, so we did<sup>not include it in any of our summary</sup> regressions. The Barro-Lee measure of<sup>terms of trade shocks (growth rate of export</sup> prices minus growth rate of import prices<sup>averaged over the preceding 5-year period) was</sup> not significant in any of our regressions.

# **_Civil disturbances, wars, and famines_**

As mentioned above, in times of civil strife, armed<sup>conflicts and large-scale atrocities</sup> tend to occur away from the center of power<sup>( battlefields are usually in the countryside.</sup> Mozambique and many other war-torn countries<sup>saw refugees flock to cities, usually the</sup> main city, which is also where relief efforts<sup>tend to be concentrated. Morrison's 1993</sup> study of politically motivated violence and migration<sup>in Guatemala showed that even</sup> when the number of deaths is relatively small,<sup>the "climate of fear" that these deaths</sup> instill - particularly in rural residents - appears to<sup>have caused many individuals to</sup> migrate.

"These were calculated on an annual basis, with<sup>the variable given a value of 2 if the negative shock was</sup> at least 2 standard deviations below expected,<sup>1 if the shock was between 1 and 2 standard deviations</sup> below expected, and 0 otherwise. NEGCROPSH<sup>is the sum of these over five years. It can therefore take a</sup> value between 0, indicating no shocks in the previous<sup>five years, and 10 which would imply the country</sup> experienced severe negative shocks to crop<sup>yields every year in the past five years. POSCROPSH was</sup> calculated using the same methodology. Expected<sup>yield was calculated using OLS estimates from a</sup> regression of actual yield on time.

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