Björk talked about the song in an interview with David Hemingway:

I wrote the melody for ‘Human Behaviour’ as a kid. A lot of the melodies on Debut I wrote as a teenager and put aside because I was in punk bands and they weren’t punk. The lyric is almost like a child’s point of view and the video that I did with Michel Gondry was based on childhood memories. Have I worked out human behaviour? I guess not.

The main melodic sample heard throughout the song was taken from Ray Brown Orchestra’s 1970 song “Go Down Dying”:

It’s gibberish. Some people think it’s Icelandic, but Björk said:

I tend to want to have songs that way that there are sections which are strict always the same lyrics and train your discipline and then have sections which are space and I can do whatever I want. So every time I sing those sections, they’re different. Sometimes Icelandic words, but mostly gibberish.