adverb
Used to describe a general tendency, often with a negative or undesirable outcome. Followed by a verb phrase.
人は得てして自分の欠点に気づかないものだ。
People tend not to notice their own flaws.
忙しいときは、得てして大事なことを忘れがちだ。
When you're busy, you tend to forget important things.
とかく is similar in meaning but can also be used as a noun or する-verb, and often appears in the set phrase とかくするうちに.
ややもすれば is more literary and emphasizes a slight tendency that can easily lead to a certain result.
Derived from the verb 得る (える, 'to get') in its classical 連用形 (continuative form) 得 (え) + particle て + verb する, literally 'getting and doing', which developed into an adverbial expression meaning 'tend to'.