せい・所為sei[1]atamadaka (head-high)

1. nounconsequence; outcome; result; blame; faultoften ...のせい
   あなたの頭痛ずつうは過労かろうのせいだ。↗

   Your headache comes from overwork.

• Related理由りゆうNeutralNeutral word for a reason or cause; 「せい」 usually attributes a bad result and carries a tone of blame.
• Related根拠こんきょNeutralThe grounds or basis for a judgment or claim, as in 「根拠のない噂」; not the cause behind a result, so it cannot replace 「せい」.
• Related由よしWrittenArchaic, literary word for a reason or circumstance; also used to mean 'I hear that', as in 「ご健勝の由」.
• Related事由じゆうWrittenFormal term used in law and official documents for the reason or particulars of a matter, as in 「退職事由」.
• Related事情じじょうNeutralCircumstances or the reasons behind a situation, as in 「家庭の事情」; softer and more neutral than the blaming 「せい」.
• Related由来ゆらいNeutralThe origin or history of a thing; 「せい」 is the cause you blame for a bad result.
• Related一理いちりNeutralSome truth or a valid point in what someone says, as in 「一理ある」; it does not express a cause or blame, unlike 「せい」.
• RelatedいわれNeutralOrigin or grounds, often in the negative 「謂れのない非難」, meaning criticism with no basis at all.
• Related所以ゆえんWrittenWritten/formal prose for the reason why something is so, as in 「人の人たる所以」; read 「ゆえん」, not 「せい」.
• Related原由げんゆWrittenRare written word for a cause or origin; 「原因」 is the usual choice in everyday speech.
• Related成因せいいんWrittenThe cause behind how something formed, used mostly in geology and science, as in 「火山の成因」.
• RelatedかどWrittenWritten word for a charge or grounds, as in 「窃盗の廉で逮捕」; used for formal legal charges.

Use 「せい」 to blame a bad result on something. For a neutral reason say 「理由」; for the grounds behind a judgment or claim, 「根拠」; for circumstances, 「事情」. In formal or legal writing, 「事由」 states the reason for something and 「廉」 names a charge, as in an arrest. For origins use 「由来」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 原因げんいん、多おおくの場合ばあい、良よくない結果けっかの原因げんいん。
2. 良よくないことが起おこったことの責任せきにんの所在しょざい。