People have intellect and are rational beings, yet they very often behave in irrational ways. Psychology and social science attempt to give many explanations for human irrationality. These attempts are useful but frequently don’t have a biblical basis. The Bible does address the question of irrationality. According to Scripture, people turn away from reason because they don’t want to acknowledge the truth that they are sinners who need God. Perhaps more importantly, the Bible also addresses why we are capable of being rational in the first place. Asking “why are people irrational?” is useful. Yet the question of “why are people capable of being rational?” is a major stumbling block for the person who rejects belief in God. Ironically, we can only have confidence in human reason if God does, in fact, exist.The counter-question of “why are people rational?” is better explained first. If all of our “reason” is simply the product of unguided evolution, then we can’t actually trust it. Evolution only favors traits that enhance survival. And, as is plain to see, holding to “truth” is not always in a creature’s best interests. “Believing” is not the same as “believing.” So, if we believe that there is such a thing as Reason, we have to believe that it is non-physical and not explained by nature or naturalism. In short, either God exists, or we can’t trust our reasoning!Some people don’t want to acknowledge God, so they have to reject truth. This doesn’t mean they’re stupid or insane. But those who turn away from God have to do some mental gymnastics in order to justify their rebellion. People can be irrational over non-spiritual issues, as well. However, the basic reasons are the same. Whether it involves spiritual or non-spiritual issues, people tend to be irrational as a defense mechanism.People are capable of reason, but they are often irrational. There are four major factors affecting our application of reason: emotions, ignorance, prejudice, and preference. We sometimes respond with pure instinct, which is emotion. We may attempt to reason through an issue without all the necessary information, which is ignorance. We may dislike something and look for excuses to reject it, which is prejudice. Or we may like something and look for excuses to defend it, which is preference.The spiritual side of why people are irrational is explained in the Bible. Second Peter 2:12 is a good summary of the biblical view of irrational thinking: “These people [false teachers] blaspheme in matters they do not understand. They are like unreasoning animals, creatures of instinct.” In this verse, lack of reason is considered a characteristic of animals. Rather than use good reasoning, people who reject the truth choose to follow their instincts—and sinful human nature, followed instinctively, leads people into error. Romans 1:20–23 says more or less the same thing. There is enough evidence of God in the world that people ought to seek Him. Those who don’t are choosing to follow nonsense instead of wisdom, preferring the animalistic approach of instinct and irrationality. In doing this, they choose to maintain ignorance instead of pursue truth. The avenue of truth is available but unsought (see Matthew 7:7).At the same time, some human irrationality is based on preference and prejudice. Accepting the truth of God means admitting one’s own sin. For some, acknowledging sin and the need of a Savior is unacceptable (John 3:19–20). If people refuse to hear God’s voice, He will let them suffer the consequences (Psalm 81:12). Often, this means letting them drift into irrational, illogical thinking (1 Corinthians 2:14).So, whether or not the issue is spiritual, people are irrational for one of two basic reasons: we don’t care for the truth, or we don’t know enough to know the truth. And, most of the time, especially in spiritual matters, whether or not we care about the truth makes all the difference as to whether we’ll try to learn it (John 7:17).