(Note: This article will make more sense if you first read the 'Psych' section of this website.)
Most of us were taught that the Fall of Man was a spiritual event. But it was also a physical event that changed the DNA of every living organism. Eden was a perfect place. No predatory carnivores. No tapeworms or parasites. No deadly viruses and bacteria. No physical or mental illness. And most notably, no death. But everything changed after the Fall! The world is now full of dangerous creatures and organisms. And we have learned that death, our great enemy, is actually programmed into our DNA! It is my opinion that the Fall also affected our brains in a subtle way that supports our sin nature.
When the Bible speaks about the "heart" it is talking about who you really are in the innermost part of your being. The Bible also says that the true nature of our heart is hidden from us, and that only God knows our true heart. How it is possible that we don't really know ourselves? Modern science offers an explanation that is consistent with the Bible. The specific parts of the brain that house what the Bible calls "the heart" are the nonconscious mind and the limbic system. The limbic system is involved in emotions, memory, and motivation. The nonconscious mind is a broader cognitive function that involves various brain regions working together. Numerous psychological studies confirm that what we claim to believe versus what we truly believe in our heart are two different things. The technical name for this abberant condition is "cognitive dissonance." It is due to a "defense mechanism" of the subconcious mind that suppresses awareness to our evil traits and ungodly beliefs. The neuroscience is as follows. Thinking about our "dark side" generates stressful "feel-bad" chemicals in our brain. Therefore, our nonconscious mind, which is all about keeping our brain happy and stress-free, silences the offending brain circuits that hold this upsetting information by taking them "off the main grid." This is a neurological form of censorship, kinda like being put in FaceBook Jail. That's why it's so difficult for anyone to detect the ungodly attitudes and beliefs in their own heart through self-examination . . . even when their faults may be obvious to others.
Summary - The Biblical claim that we are unaware of the true condition of our hearts is supported by modern neuroscience. Specifically, the architecture of our Post-Fall brain actively works to conceal the evil in our hearts by "taking it off the main grid," where it can remain undetected for years and sometimes for a lifetime! Surely this was not the original Pre_Fall architecture of the human mind!