一方だ|いっぽうだ
Expresses that a situation keeps changing in one direction without stopping, often a worsening trend: "keeps ~ing," "is only getting more ~."
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Common in news, reports and everyday talk about social or economic trends such as prices, population or health, and it often carries a sense of concern. The continuative 一方で links the trend to a following clause (example 1). Unlike 続ける, which simply continues an action, 一方だ describes an ongoing one-directional change.
Do not use it with verbs that do not express change, such as 食べる or 読む; it needs a verb of increase, decrease or transformation.
Note that 動詞辞書形 + 一方で here means "keeps ~ing, and…", not the contrastive "on the other hand" sense of 一方で.
Do not attach 一方だ to the た-form; use the dictionary form, and mark past time on 一方だ itself (一方だった).
- ばかりだN2「ばかりだ」 also attaches to the dictionary form of change verbs and means 'more and more…'; in most cases it is interchangeable with 「一方だ」, but it carries a slightly stronger negative, critical tone. Note that 「たばかりだ」, attached to the た-form, means 'just did…'.
- つつあるN2「つつある」 attaches to the ます-stem (e.g., 減りつつある) and objectively describes a change that is in progress and not yet complete; it works for both good and bad trends and has a formal, written style. 「一方だ」, by contrast, emphasizes one-way development with no improvement and often carries a sense of worry.
- 一方でN3「一方で」 expresses a two-sided contrast or adds another aspect; it can attach to verbs, adjectives, and 「名詞＋である」, and can also appear at the start of a sentence. 「一方だ」 expresses continuous change, and although its mid-sentence form is also 「〜一方で」, it follows a change verb, so the two cannot be swapped.
- がちN3「がち」 indicates that a (usually negative) situation tends to happen or occur repeatedly, while 「一方だ」 indicates a change continuing in one direction. 「がち」 attaches to the ます-stem or to nouns, whereas 「一方だ」 attaches to the dictionary form of change verbs, so they generally cannot be interchanged.
- 続けるN4「続ける」 attaches to the ます-form and expresses an action continuing without interruption, possibly with a nuance of persistence; it is not limited to change verbs. 「一方だ」 attaches to the dictionary form of change verbs and emphasizes 'more and more…', often with a sense of worry. 「減り続ける」 and 「減る一方だ」 are close in meaning.
ばかりだ is a close synonym for a one-way, usually negative trend, while つつある marks a change as currently in progress without stressing that it keeps heading only one way. 一方で (contrast) sets two facts against each other, がち marks a frequent tendency rather than a continuing change, and 続ける simply means continuing an action.
