Capacity and Age

• Entering a contract with a minor, defined as a person under 18 or 21 years of age -- depending on jurisdiction -- affects consent, notes The Free Dictionary by Farlex. A minor individual does not have the mental capacity to understand the legal language and terms of a contract. For instance, a mortgage contract signed by a 12 year old isn't a valid contract as the child would not understand the legal consequences of defaulting on a loan, paying a mortgage or property taxes. Mental capacity also affects consent. Both parties must have to ability to understand the contract. Thus, when one party suffers from a mental illness or debilitating medical condition, he lacks the ability to make a reasonable decision to enter into a contract.
  
  

Communication

• Failure to read a document does not factor into consent. However, nondisclosure does invalidate a contract. Nondisclosure occurs when one party keeps quiet about an important fact that may cause the other party to not sign the agreement. An example of nondisclosure would be if a used car buyer isn't informed that the car was salvaged. The silence of one party alters the consent of the other party. Providing the other party with a fraudulent explanation makes a contract unenforceable. For instance, when explaining the terms of an agreement, an individual may tell the other party she's receiving a payment instead of making a payment.
  
  

Pressure

• Pressure, or duress, affects the validity of a contract. One party that coerces the other party into signing the contract creates an unenforceable contract. Common ways of forcing a party to agree to a contract include threatens and blackmail. Although undue influence does not fall under the category of duress, it does affect consent. Undue influence occurs when one party uses the relationship or bond with the other party to get the contract signed.
  
  

Other Factors

• Alcohol also affects consent. An intoxicated individual cannot sign a binding contract. A unilateral mistake can also void a contract, according to Nolo Press. A unilateral mistake refers to unintentional bad faith on one or both parties; for example, when one of the parties fails to disclose that payments start immediately.