Many languages lack phonemic glottal stops, but regularly insert them. For example:

English invariably inserts glottal stops before utterance-initial vowels, and often before word-initial vowels when enunciating:

/ˈɔw　ˈnɔw/ [/ˈɔwˈnɔ́ẁ/] 'Oh no!'

and also when needed to break up adjacent identical vowels:

/ði　ˈir/ [ði̠ˈʔiɰ˞] 'the ear'

Japanese is similar, except it also allows a glottal stop utterance-finally, especially in emphatic utterances:

/ee/ [ʔèéʔ] ⟨ええ?⟩ 'huh!?'

This seems to be an under-studied phenomenon despite its widespread occurrence. Do you have any references comparing how this works in different languages?