Research: Why are all humans born in sin?
  Firstly: What does “born in sin” mean?
A condensed definition: Un-redeemed, or, without conscious, redemptive re-integration into the vital blood-relationship with the Creator/Savior.
   The first answer to why “born in sin”?: The infant is solely instinct-driven. Its intent, while human, is not deliberate heart-driven intention, but auto-functions of a growing infant who will soon become a thought-driven human being. Innocent at first, there is no immediate capacity for the thoughtful reflection needed to come to an acknowledgment of a need for adjustment or forgiveness. Neglect of what is sure to become a clearly progressive obsession with instinct-driven thoughts becomes increasingly self-destructive.
  Definition of sin: [Note: Extra-biblical definitions increase as inventions of the wicked proliferate.] What we require as professing Christians is God’s definitions and His graphic illustrations to give us appropriate pause.
   Check out what Paul says right at the top of his letter to the Romans, Chapter 1 verse 28-30: And so, since they did not see fit to acknowledge God or approve of Him or consider Him worth the knowing, God gave them over to a base and condemned mind to do things not proper or decent but loathsome, until they were filled (permeated and saturated) with every kind of unrighteousness, iniquity, grasping and covetous greed, and malice. [They were] full of envy and jealousy, murder, strife, deceit and treachery, ill will and cruel ways. [They were] secret backbiters and gossipers, slanderers, hateful to and hating God, full of insolence, arrogance, [and] boasting; inventors of new forms of evil, disobedient and undutiful to parents. Don’t point, sound like someone you know?

   This obsession to live separate (isolated) from God, His Son and His Book inflicts deep-tissue wounds. Those wounds produced desperate reflections like Psalm 51 To the Chief Musician. A Psalm of David; when Nathan the prophet came to him after he had sinned with Bathsheba.

Have mercy upon me, O God, according to Your steadfast love; according to the multitude of Your tender mercy and loving-kindness blot out my transgressions. Against You, You only, have I sinned and done what is evil in Your sight.
   Paul’s conclusion to us: Romans 11:30Just as you were once disobedient and rebellious toward God but now have obtained [His] mercy…31So they also now are being disobedient [when you are receiving mercy], that they, in turn, may one day, through the mercy you are enjoying, also receive mercy [that they may share the mercy which has been shown to you & through you as messengers of the Gospel to them].
   Now, the “why?”– God’s answer: Romans 11:32For God has consigned (penned up) all men to disobedience, only that He may have mercy on them all [alike].33Oh, the depth of the riches and wisdom and knowledge of God! How unfathomable (inscrutable, unsearchable) are His judgments (His decisions)! And how untraceable (mysterious, undiscoverable) are His ways (His methods, His paths)!
Yet His “so loved,” incited grace and demanded He and His come and take up residence in the hearts of those who desire Him above all. His grace to us distributes the graces/tools we put to use to live lives transformed and ruled by His nature.