The most compelling are the biblical declarations, which trump observation because they are the word of God. The Bible says that the heart is desperately wicked, that there is no one who does not sin, and that from conception we are sinners (Psalm 51:5). This is the one universal inherited trait, passed down from Adam to each of us.
And observation agrees with the Bible. As soon as a baby is able to express itself it does so in sinful ways. Among the first words a baby learns are typically the words, “mine,” and “no,” and their actions are right in line with those words, expressing the self-centeredness and rebellion that proves all people to be children of Adam.