Acephala Syndrome

Acephala Syndrome is a condition that exclusively affects Saiyans and Saiyan hybrids, and involves a variety of symptoms all tied to a specific area in the Saiyan brain.

Overview
This condition is particularly rare among full-blooded Saiyans, and has few recorded cases throughout most of the species' history. Its specific causes have never been identified, as it has been linked to a handful of different factors. It is known to develop in rare instances of brain damage, and has even shown signs that imply it may be hereditary, though it remains undiscovered what causes a person to become a carrier of the condition.

The effects of this condition vary from person to person, but they are all linked to a specific area of the brain responsible for the situational production of various hormones and chemicals like seratonin and dopamine. This part of the Saiyan brain is responsible for their consistent and overwhelming drive for combat, and is also partially responsible for the lack of empathy present in many Saiyans born after the colonization of Planet Vegeta. The various effects are all separated into two different, distinct types, and though they all share a handful of similarities, each type affects those afflicted with them in drastically different ways.

Type-1 Acephala Syndrome
Type-1 Acephala Syndrome or T-1AS is the form of this condition that full-blooded Saiyans are most frequently affected by. Those afflicted with it are both far less aggressive and far less driven to fight than most Saiyans are, as a result of the brain failing to produce a particular set of hormones, a certain group of chemicals, and excess amounts of adrenaline when the Saiyan in question is faced with an opportunity to fight. This condition has no correlation with a Saiyan's talent for combat  — it simply keeps them from being driven to seek it out. As a matter of fact, Saiyans born with T-1AS often display signs of a magnificent talent for battle in the rare instances when they do give into their primal instincts to fight, but more often than not these displays occur in short bursts and are usually considered flukes by those who witness them.

Examples of Saiyans with Type-1 Acephala Syndrome include Gohan, Kale, Bra, and Tarble.

Type-2 Acephala Syndrome
Type-2 Acephala Syndrome  or  T-2AS is a less form of Acephala Syndrome typically linked to brain damage, and it is also frequently seen in many half-breed Saiyans due to influence from their parents. It is also widely believed to be a recessive, hereditary gene, but this has never been confirmed by any who study Saiyan biology. Saiyans with Type-2 retain a passive desire for combat unlike those afflicted with Type-1, but lack the aggressive nature of most Saiyans and the apathy present in most of the Saiyans who inhabited Planet Vegeta. These symptoms have the same cause as those present in Type-1  —  they are caused by a lack of chemical, hormone, and adrenaline production in the brain.

Examples of Saiyans with Type-2 Acephala Syndrome include Goku, Goten, Trunks, and Gine.

Trivia

 * Acephala Syndrome is named after the Acephala group of vegetables, which includes both kale and cabbage. This is a nod to Kale, who is afflicted with Type-1 Acephala Syndrome, and Cabba, who is not actually affected by the condition, but still displays many of its symptoms.
 * The condition is also a rough double pun on the term "acephalolus", which refers to a society without leaders or hierarchies. This is not incredibly significant, but mostly serves to highlight the differences between the cultures of the Saiyans living on Earth and those living on Planet Vegeta.