Specialization can boom productiveness and offer a comparative gain for a corporation or economy. A country achieves economies of scale and by attaining a greater advantage in the specialized area. 
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</h3><h3>What do you mean by Specialization?</h3>
Specialization is a way of manufacturing wherein an entity specializes in the production of a confined scope of products to benefit an extra degree of efficiency.
Microeconomic specialization includes the character actors and financial components, and macroeconomic specialization includes the large gain an economic system holds in production.
Therefore, Specialization can boom productiveness and offer a comparative gain for a corporation or economy. 
Learn more about Specialization in economics here:
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Answer:
1. War and insecurity 2. Economic crisis (poverty) 3. Overpopulation
Explanation:
1. War and insecurity - as example Syrian crisis and war against ISIS
2. Economic crisis (poverty) - mass immigration from Poland to USA in the late XIX and early XX century. 
3.  Overpopulation - mass immigration to Europe from African countries in the last few decades. 
 
        
             
        
        
        
They are based on nomadic Hunter-Gatherer societies.