Hulk is a result of Captain America’s super soldier experiment, but there are three major differences that gave Bruce Banner his monstrous alter ego.

Steve Rogers and Bruce Banner owe their superpowers to the same super soldier serum, but their superhuman transformation into Captain America and the Hulk is extremely different. Just like the comics, the MCU has a variety of super soldiers. From Red Skull to Emil Blonsky and Karli Morgenthau, many characters have attempted to reach the peak of human performance, all to different results. And yet, Steve Rogers continues to be the perfect super soldier that everyone still wants to replicate.

Bruce Banner went through a similar process, but instead of transforming into an MCU super soldier, he gained a monstrous alter ego. Banner suffered the consequences for a long time, as he couldn’t lose his calm, let his heart rate raise, or even take his own life without unleashing the Hulk. Even after making peace with his monstrous counterpart, Bruce Banner still needs to incorporate it in his daily life, and he can not go back to his original human form without a special inhibitor. There’s one major change in Bruce Banner’s attempt to recreate the super soldier serum, but two subtle details also influenced the Hulk’s birth.

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Hulk’s Super Soldier Transformation Is Powered By Gamma Radiation

The most important difference between Captain America’s and Hulk’s super soldier transformations is the use of radiation. Originally, Abraham Erskine used Vita Rays, which shot all over Steve Rogers body after he ingested the serum in Captain America: The First Avenger. The combination between Erskine’s serum formula and the Vita Rays allowed Steve Rogers’ body to develop to peak form in a matter of minutes, but the specific details of the process died with Erskine. Decades later, Bruce Banner tried to replicate the process with a copy of the original super soldier serum, but replaced Vita radiation with gamma radiation. Unfortunately, the dose Banner bombarded himself with was extremely high. This gamma overdose awakened something in Bruce Banner, and it gave birth to the famously angry Hulk.

Bruce Banner’s Psyche Affects His Super Soldier Transformation

Gamma radiation is not the only element that makes the difference between Captain America and the Hulk. After all, Emil Blonsky and Jennifer Walters also had a Hulk transformation, but they didn’t spawn an alter ego, and they are able to transform back and forth whenever they wish. Bruce Banner, on the other hand, could only transform into the Hulk when his emotions went out of control. Likewise, Banner couldn’t Hulk out after being defeated by Thanos after the Mad Titan humiliated him at the beginning of Avengers: Infinity War, and he finally managed to achieve his powerful but more contained “Smart Hulk” form only when he was able to focus on his inner peace.

In the comics, Bruce Banner’s father David Banner mistreated him throughout his childhood, causing Bruce a great deal of psychological trauma, which in turn informed the Hulk’s instinctual desire to protect Bruce Banner and personify his pent-up anger. If Bruce Banner’s backstory is the same in the MCU, it’s possible that part of his unconscious mind had already constructed the Hulk as a defense mechanism before he received the super soldier serum, and the Gamma radiation only allowed the monster to manifest itself.

Hulk May Be An Entirely Different Being Altogether

While the Hulk may be part of Bruce Banner’s unconscious mind come to life, the Jade Giant doesn’t always get along with the scientist. Bruce Banner always refers to Hulk as “the other guy,” and Thor: Ragnarok started to showcase the divide between the two. Hulk’s successful career as a gladiator in Sakaar convinced the monster to stay for at least two years, and when Banner finally managed to come back, he didn’t know about the Hulk’s extraterrestrial exploits. Soon after, Thanos beat up the Hulk, after which he completely refused to come out again. This could be read as Banner’s unconscious mind reacting to his own defeat, but it’s more likely proof that Hulk is already an independent being with his own fears and wishes. Now, this makes Bruce Banner’s Smart Hulk a questionable experiment, as Banner took over Hulk’s body but retained his human mind. If the original Hulk is still somewhere inside Bruce Banner’s mind, he may not have liked being thrown to the wayside, and he may be planning to come back with a vengeance.

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Steve Rogers is much more stable than the Hulk, but he lacks Hulk’s power. Abomination and She-Hulk are close to Hulk in regard to power, but they lack his rage. Although Hulk is a direct result of the original Captain America‘s super soldier experiment, he’s an entity of his own by this point. Gamma radiation, Bruce Banner’s subconscious mind, and Hulk’s own personality have created an independent being that may or may not continue to hide inside Banner’s mind. If Hulk‘s MCU story continues in the form of a World War Hulk movie, the Green Goliath is likely to evolve into an even angrier self-governing monster.