Goku is an anomaly because all Saiyans are naturally evil. However he was dropped on his head as a child, and was good from then on. Goku was the first Saiyan to turn Super Saiyan in over 1000 years. Goku is also the only pure Saiyan to turn Super Saiyan 3 (Gotenks also turns Super Saiyan 3, but is the result of two Halfbreed Saiyans fused).
Goku's childhood is detailed in the original Dragon Ball series.
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