When a company reports results that surprise the market, the price reacts immediately. In an efficient market that reaction would be complete, and what followed would be random.<\/p>\n\n\n\n
It hasn’t been. Prices have historically continued drifting in the direction of the surprise for weeks afterwards, a pattern documented since 1968 and given the label post-earnings announcement drift. It survived publication, survived decades of attention, and was described by one of the most prominent efficient markets researchers as the granddaddy of underreaction events.<\/p>\n\n\n\n
It’s now weakening, and the argument about why is more interesting than the pattern itself.<\/p>\n\n\n\n
Among the <\/a>behavioural finance concepts<\/a> with the longest research record, this one has the clearest link between a specific cognitive tendency and an observable price pattern.<\/p>\n\n\n\n The standard explanation runs through several linked ideas:<\/p>\n\n\n\n That last one is what turns a bias into a persistent pattern. A tendency only survives in prices if correcting it is costly enough that professionals don’t fully do so.<\/p>\n\n\n\n The magnitude has declined substantially, and the usual explanation is that arbitrageurs found the signal and traded it away.<\/p>\n\n\n\n Recent research disputes that as the whole story. Examining the attenuation directly, one study offers an alternative, finding that declining persistence in earnings<\/a> news seems to be just as important as, if not more important than, arbitrage trading in explaining declining drift.<\/p>\n\n\n\n The reasoning is subtle. The drift existed partly because investors failed to appreciate that firms with good current earnings news tended to have good news again next quarter. If that persistence in earnings itself has weakened, then the thing investors were failing to anticipate is no longer there to be anticipated.<\/p>\n\n\n\n On that reading, the anomaly didn’t get arbitraged away. Its underlying cause changed.<\/p>\n\n\n\n A more fundamental challenge questions whether underreaction explains the pattern at all.<\/p>\n\n\n\n One analysis argues that while aggregated portfolio returns do drift in a way consistent with underreaction theory, the question of why earnings surprise portfolios drift remains open, and the underreaction theory incorrectly describes close to half of the observations.<\/p>\n\n\n\n That’s a serious objection. An explanation that fits the average while misdescribing half the individual cases may be capturing something other than the mechanism it claims.<\/p>\n\n\n\n Alternative explanations exist in the literature, including compensation for liquidity risk, arbitrage risk, and information uncertainty. Some research argues the relationship with limits to arbitrage disappears once expected growth risk is properly controlled for.<\/p>\n\n\n\n Whatever the mechanism, the conditions under which it’s most pronounced are consistent:<\/p>\n\n\n\n The last point is where recent work has focused. Numerical surprises get priced efficiently as infrastructure improves. Information requiring interpretation continues to be absorbed slowly, which suggests the bottleneck is reading comprehension rather than data access.<\/p>\n\n\n\n There’s a further wrinkle. Research on firms with actively traded options, which are the easiest to arbitrage, has found the correction can overshoot, with unsophisticated arbitrageurs overcompensating for the documented underreaction. The biases that create an anomaly can apply equally to the people trying to exploit it.<\/p>\n\n\n\n The conditions that make the drift largest are precisely the conditions that make trading it expensive. That correlation isn’t incidental, it’s the reason the pattern persisted<\/a>, and it means the academic returns are unlikely to survive real costs for an individual.<\/p>\n\n\n\n What’s useful is the underlying observation. Markets absorb information at different speeds depending on how hard it is to interpret, and the delay is longest exactly where fewest people are looking.<\/p>\n\n\n\n For someone holding a company through a results announcement, that suggests the initial price reaction may be incomplete, particularly in smaller or less covered names, and that the fuller picture emerges over subsequent weeks rather than on the day.<\/p>\n\n\n\n The most instructive thing here isn’t the anomaly. It’s watching a well-documented pattern get explained, challenged, re-explained and partially dissolved over five decades of scrutiny.<\/p>\n\n\n\n Underreaction remains the most consistent explanation across the literature. It’s also contested, incomplete, and possibly measuring something adjacent to what it claims. Holding a finding and its objections together is closer to how the evidence actually looks than any single confident account of it.<\/p>\n","protected":false},"excerpt":{"rendered":" When a company reports results that surprise the market, the price reacts immediately. In an efficient market that reaction would be complete, and what followed would be random. It hasn’t been. Prices have historically continued drifting in the direction of the surprise for weeks afterwards, a pattern documented since 1968 and given the label post-earnings…<\/p>\n","protected":false},"author":2,"featured_media":3228,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_kad_blocks_custom_css":"","_kad_blocks_head_custom_js":"","_kad_blocks_body_custom_js":"","_kad_blocks_footer_custom_js":"","_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"_kad_post_classname":"","footnotes":""},"categories":[43],"tags":[],"class_list":["post-3227","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blogs"],"taxonomy_info":{"category":[{"value":43,"label":"Blogs"}]},"featured_image_src_large":["https:\/\/www.fontmirror.com\/en\/wp-content\/uploads\/2026\/09\/Post-Earnings-Drift.png",750,500,false],"author_info":{"display_name":"Kokou Adzo","author_link":"https:\/\/www.fontmirror.com\/en\/author\/kokou\/"},"comment_info":0,"category_info":[{"term_id":43,"name":"Blogs","slug":"blogs","term_group":0,"term_taxonomy_id":43,"taxonomy":"category","description":"","parent":0,"count":38,"filter":"raw","cat_ID":43,"category_count":38,"category_description":"","cat_name":"Blogs","category_nicename":"blogs","category_parent":0}],"tag_info":false,"_links":{"self":[{"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/posts\/3227","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/comments?post=3227"}],"version-history":[{"count":1,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/posts\/3227\/revisions"}],"predecessor-version":[{"id":3229,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/posts\/3227\/revisions\/3229"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/media\/3228"}],"wp:attachment":[{"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/media?parent=3227"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/categories?post=3227"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.fontmirror.com\/en\/wp-json\/wp\/v2\/tags?post=3227"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
<\/a>Why the Drift Has Faded<\/strong><\/h2>\n\n\n\n
<\/a>The Dissenting Reading<\/strong><\/h2>\n\n\n\n
<\/a>What Makes the Drift Larger<\/strong><\/h2>\n\n\n\n
\n
<\/a>Why This Matters Without Being Tradeable<\/strong><\/h2>\n\n\n\n
<\/a>What the Debate Actually Teaches<\/strong><\/h2>\n\n\n\n