Credit

Credit Ld r

• An agent is a potential lender if it is too old to have children, in which case the maximum amount it may lend is one-half of its current wealth; • An agent is a potential lender if it is of childbearing age and has wealth in excess of the amount necessary to have children, in which case the maximum amount it may lend is the excess wealth;

• An agent is a potential borrower if it is of childbearing age and has insufficient wealth to have a child and has income (resources gathered, minus metabolism, minus other loan obligations) in the present period making it credit-worthy for a loan written at terms specified by the lender;

• If a potential borrower and a potential lender are neighbors then a loan is originated with a duration of d years at the rate of r percent, and the face value of the loan is transferred from the lender to the borrower; • At the time of the loan due date, if the borrower has sufficient wealth to

repay the loan then a transfer from the borrower to the lender is made; else the borrower is required to pay back half of its wealth and a new loan is originated for the remaining sum;

• If the borrower on an active loan dies before the due date then the lender simply takes a loss;

• If the lender on an active loan dies before the due date then the bor- rower is not required to pay back the loan, unless inheritance rule I is active, in which case the lender’s children now become the borrower’s creditors.

The Credit rule states that after an agent’s death all outstanding loans that this agent owes are nullified. This is reasonable as, on death, all the dying agents resources are destroyed. If, however, the inheritance rule is also in play then the children of the dying agent inherit their wealth. If an agent dies with outstanding loans due to them then these loans are passed on to their children and any money owed to them now become owed to their children.

It is not stated what happens to loans that the dying agent owes to other agents. While, logically, we might deduce that these debts are now debts of its children, in the absence of information to the contrary we have assumed that these debts are now nullified even though this arguably makes less sense in the light of the rest of the inheritance rule.

Similarly, it is unclear how loans belonging to agents killed in combat are han- dled. Some fraction of their resources are taken by the aggressor agent that killed them but it is not stated whether we redistribute any remaining resources (if any remain) or for that matter loans and debts. Does the combat rule override the in- heritance rule? We have assumed it does.

In both these cases the ambiguities are caused by the interaction between dif- ferent rules. In isolation each rule seems clear but when used together they are not. As ABM are used to study emergent properties it should be no surprise that the interactions between rules can cause ambiguities. This is especially true in Sugarscape as it contains a large number of rules that can be used in many differ- ent combinations. We also note that the process of formally specifying these rules brings these ambiguities to the fore where they can be properly addressed.

Creditworthiness depends on outstanding loans and ability to repay them. We have based this on all outstanding loans no matter when due. This is the simplest and was probably the intention as no information on how else it might be calculated is provided.

We also note that when we pay back loans before we run the death/replacement rule then there is the possibility of divide by zero. This would occur as the agent would have no resources. It could be argued that this problem is due to our having a separate Death rule. We have avoided this possible error by imposing restrictions on the order of the rule execution that precludes this possibility.

The Credit rule nowhere states how much an agent can or should loan to a potential borrower. Does an agent loan as much as it has available or only a fixed percentage of its resources? A borrower is defined as an agent that is fertile and that has insufficient wealth to have children. We have two issues here.

First, there is no mention of any minimum amount required by agents to have children in the Mating rule. This represents an important piece of missing informa- tion from the Mating rule. As a result, we do not know what this minimum amount should be.

Secondly, is this amount the minimum amount looked for by a borrower or a maximum amount a borrower is prepared to take? We have assumed that the difference between the resources required and held is the amount required by the borrowing agent. We also assume that an agent will accept a loan for a lesser amount if that is all that is offered and that the maximum amount offered by a lender is no more than that required by the borrower to have children.

As with other rules it is not stated how credit operates if there are multiple resources in play so we have assumed that separate loan agreements can be made for each resource.