Selectorate Theory is a political theory that attributes the actions of leaders to rational, calculated decisions based on acquiring and maintaining power, originally proposed by Bruce Bueno de Mesquita.

History 
Originally proposed in 1993 as a part of a work titled The Logic of Political Survival, co-written by Bruce Bueno de Mesquita, Alastair Smith, Randolph M. Siverson, and James D. Morrow, published by MIT Press. It discusses the selectorate theory of politics. Their theory was refined into a 2011 publication titled The Dictator's Handbook. The main thesis of the book is the implications of selectorate theory and how to improve world and national politics based on its principles.

Main Ideas

No Ruler rules alone 
The theory is based on a single idea: No political leader has ever had total control of their government. Leaders need the support of at least some people to lead, because those people either do something for the leader or tell other people what to do. This means that there is a group of people the leader needs to persuade in order to obtain or keep power. These people are the selectorate, and are divided into three categories:

The Three Dimensions of Politics 
There are three groups of people that a leader, or their contenders, must remain aware of in order to keep, or acquire, power:

 The Essentials are the people whose support is absolutely necessary for a person to come to power and keep power.
 The Influentials are the people who, though are important for a leader to gain power, aren't always the people that determine whether the leader stays in power.
 The Nominals are the people who have some influence over who is in charge, but aren't necessary for a leader to obtain power.

There are many ways that a leader and the selectorate interact with each other, mostly through the five rules that leaders follow to acquire and keep power.

The Five Rules to Rule By 

 A ruler wants to keep the amount of Essentials they need to rely on as few as possible.
 A ruler wants to keep the group of Nominals as high as possible, so they can replace disloyal Essentials and Influentials.
 A ruler must have some way of paying their selectorate through taxes and a treasury.
 A ruler must pay their Essentials enough to stay in power, but not more.
 A ruler cannot take money away from their Essentials to make people's lives better.

The Two Strategies 
There are two methods of governance that form using the Selectorate Theory: a government with few Essentials- called a small coalition- and a government with a large group of Essentials- called a large coalition.

Small coalition governments are autocracies such as North Korea or the Soviet Union. Although many people can vote in their elections, if there are any, a leader stays in power because of the very few people who are rewarded for their loyalty. In small coalition governments, a leader can simply bribe their Essentials with material rewards such as money or real estate.

Large coalition governments are governments that force leaders to appeal to large amounts of people to take power, such as through an election. This style of governance makes it implausible to bribe individual voters or Essentials, meaning a leader instead pools the money earned from taxes to create policy that disproportionately rewards their Essentials more than other groups.

How the selectorate affects leadership 
With the basic idea of how a leader and their selectorate interact with each other, there are several ways that this manifests in domestic and foreign policy.

The Fall of Monarchy 
Monarchy is a system where a regent has a small court of lords whom support the ruler. However, those who are eligible to become a member of the court is also small. This means a contender can easily promise big rewards for those who are able to become members of the court who aren't already, while those in the court must always prepare to be replaced, meaning they are never as loyal as they could be.

As such, democracy and autocracy are more stable systems for a leader because these systems allow them to follow the Second Rule: keep the Nominal selectorate as large as possible.

Foreign Aid 
Foreign aid with no strings attached, such as the giving of money from a wealthy nation to a less fortunate one, often acts as a symbiotic relationship between dictators and democrats. Large-coalition governments send foreign aid money to impoverished nations as a policy concession- thus retaining power- while the impoverished nations, who are often governed by small coalitions, distribute the money among the coalition instead of benefiting the people.

References 

Political systems
Non-fiction books