Instinctual means almost the same as instinctive: coming from instinct, not from thought. Though actually, this one belongs more to psychology and biology. Writers use it for drives and behaviours that look as if they come with the animal — hunger, fear, the urge to protect a child. Instinctive is the everyday word for a quick, unthinking reaction (an instinctive grab). Many people treat the two as twins, and you will be understood. Instinctual just sounds more like a lecture or a paper. You will not hear it much in the street, and you rarely need it if instinctive already does the job.

本能的（偏心理、生物用語，跟 instinctive 很接近；日常比較常說本能的）