Consider the following sentence:

彼女は緊張すると、重大なミスをする。
かのじょ　は　きんちょう　すると、　じゅうだいな　ミス　を　する。
She makes major mistakes when she gets nervous.
Literally: “she + は + nervousness + do + と, + serious / grave + mistake + does.”

That's a pretty straightforward sentence, though perhaps some of you out there are not familiar with this usage of と. We'll look at it in another lesson, but the basic version is this: "Phrase 1 と Phrase 2" means that Phrase 2 definitely occurs if/when Phrase 1 occurs.

We're focusing on something different for this lesson.

Specifically, how do you think these two sentences differ in meaning:

彼女は緊張すると、重大なミスをする。
彼女は緊張すると、重大なミスをしがちだ。

If you don't know, it's because we haven't had a lesson on ～がち yet!

Don't worry, we'll look at ～がち next month.

I mean right now:

JLPT N3: ～がち (prone to ~; liable to ~; tending to ~)

No more suspense, here's the difference between our two phrases:

彼女は緊張すると、重大なミスをする。
かのじょ　は　きんちょう　すると、　じゅうだいな　ミス　を　する。
She makes major mistakes when she gets nervous.
Literally: “she + は + nervousness + do + と, + serious / grave + mistake + does.”

彼女は緊張すると、重大なミスをしがちだ。
かのじょ　は　きんちょう　すると、　じゅうだいな　ミス　を　しがち　だ。
She tends to make major mistakes when she gets nervous.
Literally: “she + は + nervousness + do + と, + serious / grave + mistake + prone to do + だ.”

When we want to show that someone or something has a tendency to be a certain way, then we can use ～がち.

～がち
prone to ~; liable to ~; tending to ~

👷 Construction 👷

There are two ways that we can construct ～がち.

It can attach to the masu-stem of a verb or to a noun:

Masu-stem + がち
NOUN + がち

Verb
→ Masu-form
→ → Masu-stem
→ → → Masu-stem + がち

する (to do)
→ します (do)
→ → し-
→ → → しがち (prone to do; tending to do)

ある (to be; to have)
→ あります (is; has)
→ → あり-
→ → → ありがち (prone to be/have; tending to be/have)

Noun
→ Noun + がち

病気 (illness; sickness)
→ 病気がち (prone to be ill; tending to be sick)

Tricky Stuff

I sometimes get confused by ～がち because when we say "is prone to X," maybe X has never happened before.

But when we say "tends to X," then X has most likely happened many times, right?

～がち can mean both of these. Compare the sentence we already saw, in which "she" presumably has made big mistakes as a result of being nervous in the past:

彼女は緊張すると、重大なミスをしがちだ。
かのじょ　は　きんちょう　すると、　じゅうだいな　ミス　を　しがち　だ。
She tends to make major mistakes when she gets nervous.
Literally: “she + は + nervousness + do + と, + serious / grave + mistake + prone to do + だ.”

Accordingly, I translated it as "tends to."

Compare that with the more general phrase that follows:

怒られるのが怖くて嘘をつくのは、子供にありがちな行動です。
おこられる　の　が　こわくて　うそ　を　つく　の　は、　こども　に　ありがち　な　こうどう　です。
Children are prone to lie because they are afraid of getting in trouble.
Literally: “have someone get angry at you + のが + frightening / being scared (and) + tell a lie (=lie をつく), + children + に + prone to have + behavior + です.”

At the risk of confusing myself (and you), I will say that the above translation is more accurate than this one:

Children tend to lie because they are afraid of getting in trouble.

With this "tend to," it sounds like this is the primary reason that children lie, does it not? But that's not necessarily the case with ～がち. This fuzzy line between between "tend to do" and "prone to do" confuses me, and as a result I'm not a big fan of translating sentences with ～がち in them. ^_^

While we're on the topic of tricky translations, consider that a more literal translation of our sentence would look like this:

怒られるのが怖くて嘘をつくのは、子供にありがちな行動です。
おこられる　の　が　こわくて　うそ　を　つく　の　は、　こども　に　ありがち　な　こうどう　です。
Lying is a behavior that children are prone to do because they are afraid of making people angry.
Literally: “have someone get angry at you + のが + frightening / being scared (and) + tell a lie (=lie をつく), + children + に + prone to have + behavior + です.”

That sounded really wordy and unnatural to me, though, so I went with:

Children are prone to lie because they are afraid of getting in trouble.

These are some of the reasons that translating makes my head spin. Simultaneously, these are some of the reasons I never get tired of translating.

Note that the newly-formed word ending in ～がち acts as a na-adjective or a noun.

This means that it will either be followed by です, だ, or "な + NOUN."

Consider what we've seen so far:

しがちだ
ありがちな行動

In the following sentence, it's followed by です：

わたしはお酒を飲むと、酔っぱらって友達に迷惑をかけてしまいがちです。
わたし　は　おさけ　を　のむ　と、　よっぱらって　ともだち　に　めいわく　を　かけて　しまいがち　です。
When I drink alcohol, I tend to get drunk and cause trouble for my friends.
Literally: “I + は + alcohol + を + drink + と, + get drunk (and) + friends + に + cause trouble for (them) (and) [=annoyance を put on (them)] + am prone to end up doing + です.”

～がち is quite a common piece of grammar, and I highly encourage you to try using it in daily speech.

It might help to also pay attention to a handful of words that frequently appear with ～がち, noting that such words don't always translate cleanly into "prone to X" or "tending to X."

One example of this is:

病気がち
びょうき　がち
proneness to being ill; proneness to disease; sickly; being sick often

Let's see it in a sentence:

出産後の妻はとかく病気がちだった。
しゅっさんご　の　つま　は　とかく　びょうき　がち　だった。
My wife often got sick after having the baby.
Literally: “after giving birth + の + wife + は + being apt/prone to + sickness + がち + was.”

Oh, note that とかく often pairs up with ～がち. This doesn't really change the meaning of the sentence at all, so I wouldn't worry about using it in your own sentences.

Finally, here are some other words that commonly appear with ～がち：

忘れがち（わすれがち // forgetful; tending to forget）
怠けがち（なまけがち // tending to be lazy）
遅れがち（おくれがち // tending to be late）

Happy studies, yo.