The reliability of project estimates depend on a number of exogenous and/or endogenous factors to the estimator. Construction's effort to improve the estimating function has been addressed at the exogenous factors. Notwithstanding progressive improvement achieved in estimating from addressing such exogenous factors, most project-oriented industries still suffer from unreliable estimates. Understanding the root causes of the persistence of unreliable estimates would therefore, call for a focus on factors other than the exogenous ones that most improvement efforts have addressed and formed the basis of the study that led to this thesis. The evidence suggests that an estimator's personality archetype has a significant influence on their estimating reliability. The finding on effect of personality goes to explain why different estimators with the same or similar experience and qualification, exposed to the same set of information, deliver different levels of reliability in estimating.