The Isolation Effect – Arizona Forensic Services, LLC

Snap Shot

The question: Is an item that stands out from its surroundings remembered better, and does that advantage come at the expense of memory for everything else, including a perpetrator’s face?

What the research shows: An item that differs from the items around it is generally recalled better than the same item in a uniform setting (the isolation or von Restorff effect), and the advantage is best explained by the processing of difference against a background of similarity. Unusual objects can draw attention in a scene and, in some studies, reduce memory for the person holding them, as weapons can.

What it does not show: Distinctiveness does not by itself impair memory for surrounding details; costs to neighboring material appear mainly with emotionally arousing items and are inconsistent. In some studies, unusual-object effects have been clearer for descriptions than for lineup identifications, and no study can say whether a particular witness’s memory for a face was impaired.

The law: Rule 702 and Houser govern expert testimony, and Arizona permits general testimony on factors affecting memory (Chapple, Nordstrom). Attention at the time of the event is one of the identification factors Arizona jurors are told to consider.

Issues for each side: Prosecutors can rely on the research to explain why a witness accurately remembers a distinctive feature and should not treat that memory as validating the rest of the account; defense counsel can examine what competed for a witness’s attention without treating an unusual object as proof that an identification is mistaken.

People tend to remember the thing that stands out. In a list of words printed in black, the one word printed in red is more likely to be recalled; in a list of nonsense syllables, a single number is more likely to be recalled than the syllables around it. This is the isolation effect, often named the von Restorff effect for Hedwig von Restorff, who described it in 1933. The effect depends on contrast: an item is memorable not because of what it is in itself, but because it differs from what surrounds it.

The effect reaches court because events often contain something distinctive: an unusual garment, a tattoo, a strange object, or a weapon. Distinctiveness can support accurate memory for that feature. The research on unusual objects and weapon focus also raises the question whether the feature that draws attention leaves less attention for other details, such as the perpetrator’s face. Both points matter, and they cut in different directions.

This paper describes the classic effect and its explanations, distinctiveness that arises from emotional and bizarre material, distinctive faces and features, and the research on unusual objects and weapon focus. It then addresses the limits of the evidence and the federal, military, and Arizona law that the topic touches. The companion paper Memory for Violence and Weapon Focus covers weapon focus itself in depth.

What the Isolation Effect Is

Von Restorff (1933) showed that when one item in a list differs in kind from the others, for example a number among syllables, memory for that item is enhanced relative to the same kind of item embedded among similar items. A review three decades later found the effect across many materials and procedures (Wallace, 1965), and a recent historical review placed it among the best known memory phenomena of the twentieth century (MacLeod, 2020). The effect is relative: the same item can be ordinary in one list and distinctive in another, depending on the background.

The effect is clearest in free recall. In recognition tests, which ask whether an item was seen before and so resemble the task a witness faces in a lineup, isolation effects have often been small or absent. A recent series of experiments confirmed the recall advantage, found no recognition advantage under traditional designs, and then found isolation effects in both recognition and memory for the source of an item once the design reduced measurement error and controlled the order of testing (Kamp et al., 2026). The recall advantage is well established; the recognition advantage is real but smaller and more dependent on how memory is tested.

Why It Happens: Distinctiveness Accounts

The intuitive explanation is that the odd item captures attention because it is surprising, and attention improves encoding. Attention is part of the story, but it is not the whole of it. In a reexamination of von Restorff’s original experiments, Hunt (1995) pointed out that she had obtained the effect even when the isolated item appeared early in the list, before a background against which it could be surprising had been established. Hunt argued that distinctiveness is better understood as a process: the isolated item is remembered well because, at study and at test, its difference from a background of similar items is registered and used.

Hunt and McDaniel (1993) developed this view into an account in which good memory depends on two kinds of processing: processing of what items have in common, which organizes them, and processing of what is unique to each, which makes each one discriminable at retrieval. Distinctiveness, on this view, is difference in the context of similarity, and neither kind of processing works well alone. Schmidt (1991) distinguished distinctiveness relative to the immediate context, as in the classic isolation effect, from distinctiveness relative to a person’s general knowledge, such as an unusual or bizarre item, and from emotional distinctiveness, and argued that these do not all operate through the same mechanism. The details remain debated, but the core finding is not: items that contrast with their context are generally remembered better than items that blend in.

Emotional and Bizarre Items

Bizarre material shows the same dependence on context. In studies of bizarre imagery, sentences describing bizarre images were recalled better than sentences describing common images when the two were mixed in the same list, but the advantage was reduced or absent when lists contained only one kind (McDaniel & Einstein, 1986). Bizarreness helps memory mainly when it stands out from ordinary material.

Emotional items raise a separate question: whether the memorable item costs memory for its surroundings. In a study in which a picture of a nude model appeared among pictures of clothed models, the nude was highly memorable but impaired memory for background details in the same picture and for pictures that followed it; a clothed picture placed among nudes was also distinctive and well remembered but produced neither cost (Schmidt, 2002). The author concluded that distinctiveness accounted for the good memory but not for the impairment, which appeared to follow from the arousing content. A broader account holds that emotional arousal strengthens processing of whatever has priority at the moment, whether because it is salient or because it matters to the person’s goals, and weakens processing of lower-priority information (Mather & Sutherland, 2011). Older list studies reported that items given special priority impaired recall of the items just before them (Tulving, 1969), but later experiments found that impairment absent or modest (Guynn & Roediger, 1995). The fair summary is that distinctiveness reliably helps memory for the distinctive item, while costs to surrounding material are less consistent and appear mainly when the item is arousing or demands attention.

Distinctive Faces and Features

Distinctiveness also operates on faces. Faces rated as unusual were recognized better than faces rated as typical in early experiments (Light et al., 1979), a finding consistent with the general principle. A distinctive feature can, however, work against identification. In a lineup study in which the perpetrator had a black eye, the distinctive feature reduced witnesses’ ability to discriminate the perpetrator from innocent lineup members whether the feature was replicated on the fillers or removed from the suspect (Jones et al., 2020). Witnesses may encode the striking feature at the expense of the face that carries it, and a feature that is accurately remembered does not establish that the face was well remembered. Methods for handling distinctive features in lineups are addressed in the companion paper on lineup construction.

Unusual Objects and Weapon Focus

The most important forensic application of distinctiveness concerns objects. Weapon focus is the finding that a visible weapon can reduce memory for the person holding it, and one leading explanation is that weapons are unusual in most settings and therefore draw attention (Lane & Houston, 2021, pp. 109–111). Several studies have tested this by showing a harmless but unusual object in place of a weapon. In two experiments in which college students watched a videotaped scene in which a man brandished nothing, a stalk of celery, or a gun, memory for the man’s appearance was reliably poorer in the celery condition in both experiments, while a gun produced the traditional weapon focus effect only in the second; self-reported emotional reactions bore little relation to memory, and the authors suggested that “weapon focus” may describe a more general attentional effect (Mitchell et al., 1998). In videotaped scenes in which a man held objects that varied independently in threat and unusualness, including a raw chicken, unusualness but not threat reduced the accuracy of witnesses’ descriptions, while identification accuracy in a photo lineup did not differ among conditions (Pickel, 1998). Weapons have also produced little memory impairment when they fit the setting or the person, such as a gun at a shooting range or carried by a police officer (Pickel, 1999).

Threat appears to contribute as well. In a slide-show study in which the target held a gun, an unusual non-threatening object (reported as a feather duster), or a neutral object, both the gun and the unusual object slowed responses on a secondary task, a sign that both drew attention, but recognition of the target’s appearance was poorest in the gun condition once confidence was taken into account (Hope & Wright, 2007). A meta-analysis found that weapon effects were larger in threatening than in non-threatening scenarios and that non-threatening unusual objects also impaired memory (Fawcett et al., 2013). Recent eye-tracking work with moving scenes adds a qualification: an unusual object, a plunger, reduced looking at the perpetrator, but a knife did not, and memory deficits for either object appeared only when the video included a soundtrack (Körner et al., 2024). The research thus supports unusualness as one cause of weapon focus, identifies threat as another, and shows that the effects depend on exposure time, context, and how memory is measured.

Limits of the Evidence

Most isolation research uses word lists or pictures studied by university students, with distinctiveness created by color, category, or content in a controlled sequence. Real events are not lists, and what stands out in an event depends on what the witness expected and attended to, which is rarely known afterward. Studies of unusual objects use brief videos or slides, mild emotional stakes, and laboratory definitions of unusualness, and in some studies their effects have been clearer for descriptions than for lineup identifications (Pickel, 1998). Some recent studies have not found the expected attentional effect for weapons. The finding that a striking item impairs memory for its neighbors is less consistent than the finding that the striking item itself is remembered well. Group findings describe tendencies under particular conditions; they cannot establish that a particular witness’s attention was captured, or that a particular face was or was not well remembered.

Questions to Ask About This Research

When an expert says that an unusual object, rather than its threat, causes weapon focus, counsel should ask which studies are relied on and what they measured: Mitchell et al. (1998) found that a stalk of celery impaired memory for the man’s appearance, Pickel (1998) found that unusualness affected descriptions but not lineup accuracy, and Hope and Wright (2007) found that an unusual non-threatening object (reported as a feather duster) drew attention but impaired memory less than a gun. Counsel should also ask whether the object in this case was unusual for the setting or for the person carrying it, how long the witness could see the person, and whether the claim concerns a description or an identification (Fawcett et al., 2013). When the classic list findings (von Restorff, 1933; Hunt, 1995) are offered, counsel should ask whether they were obtained in recall or recognition, because recognition effects are smaller and depend on design (Kamp et al., 2026). When accurate recall of a striking feature is offered as support for the rest of the account, counsel should ask what the research predicts for the surrounding details, including the face. These questions are explained in “Reading Research Claims: A Guide for Attorneys” on this site.

The Isolation Effect and the Law

Federal and Military Standards

Expert testimony about memory is governed by Federal Rule of Evidence 702, as amended December 1, 2023, under which the proponent must show the court that it is more likely than not that the rule’s requirements are met, including that the opinion reflects a reliable application of reliable methods to the case. Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993) made the trial judge a gatekeeper, and Kumho Tire Co. v. Carmichael (1999) extended that obligation to all expert testimony, including testimony based on technical or other specialized knowledge. In courts-martial, Military Rule of Evidence 702 tracks the federal rule, the 2023 federal amendment took effect in the military rule in 2025 through Military Rule of Evidence 1102 (Joint Service Committee on Military Justice, 2025), and practitioners should confirm current text; military judges apply the six factors of United States v. Houser (1993). Testimony on distinctiveness and unusual objects is ordinarily general and educational: it explains how attention and contrast affect memory and leaves to the factfinder whether a particular witness’s memory was affected.

Arizona

Arizona Rule of Evidence 702 adopted the federal formulation and the Daubert standard effective January 1, 2012, and was amended effective January 1, 2024, to conform to the 2023 federal amendment (Ariz. Sup. Ct. Order R-23-0004). State v. Chapple (1983) is Arizona’s leading case on expert testimony about eyewitness memory; excluding such testimony was an abuse of discretion on its facts, while admission otherwise remains within the trial court’s discretion, and State v. Nordstrom (2001) approved limiting an expert to general factors without opinions on the accuracy of a particular witness. Arizona’s identification instruction directs jurors to consider, among other things, the witness’s opportunity to view and degree of attention at the time of the crime and the accuracy of any prior description (RAJI (Criminal) Standard Criminal 45), factors to which research on distinctive features and unusual objects is directly relevant.

Special Issues in Legal Practice

The principles described above apply with equal force to every party in a legal proceeding. The science operates the same way regardless of who is asking the questions, and the research summarized here does not by itself favor either side. What differs between the parties is not the science but the vantage point — the particular places where these issues tend to surface, and the particular errors each party is best positioned to guard against. The two summaries below identify those recurring issues. They are offered for orientation, not as litigation strategy; in every case the controlling question is what the evidence in that case will, and will not, support, and the expert’s role is to inform the trier of fact rather than to advocate for either side.

Special Issues for Prosecutors

The research supports several propositions prosecutors can legitimately present. Distinctive features are generally well remembered, so a witness’s accurate, specific memory for an unusual garment, tattoo, vehicle, or object is consistent with how memory works. In some studies the effects of unusual objects have been clearer for descriptions than for lineup identifications, and some studies have found no reduction in identification accuracy. In one study, a weapon produced little impairment when it fit the setting (Pickel, 1999); whether the same holds for witnesses with weapons experience has not been well tested. Prosecutors are well positioned to document what the witness could see, for how long, and from where, and to obtain a description before any identification procedure.

The research also counsels caution. A vivid, confident memory of a distinctive object does not show that the witness remembered the face of the person holding it, and research on unusual objects and distinctive facial features indicates that the face may have been encoded less well. Where an unusual object or weapon was present and exposure was brief, an identification gains value from independent corroboration and from a procedure that handles any distinctive feature appropriately. Testimony or argument that treats accurate memory for the striking feature as validating the entire account goes beyond what the research shows.

Special Issues for Defense

The research supports several propositions defense counsel can legitimately present. Unusual objects and weapons can draw attention in a scene, and in a number of studies they reduced memory for the appearance of the person holding them, with the meta-analytic effect larger when the situation was threatening; the weapon effect is reliable in laboratory meta-analyses, larger for descriptions than for lineup identifications, and smaller in studies of actual crimes (see “Memory for Violence and Weapon Focus”). A distinctive facial feature can impair a witness’s ability to tell the perpetrator from innocent lineup members. Evidence of what competed for the witness’s attention, how long the person was visible, and whether a distinctive feature appeared only on the suspect is a legitimate basis for examining an identification, and expert testimony on these principles can help the factfinder weigh it.

The research also counsels caution about the limits that apply to the defense. The effects are inconsistent, in some studies clearer for descriptions than for identifications, and absent in some recent studies, and the presence of an unusual object does not establish that a particular witness failed to encode the perpetrator’s face. Distinctiveness also supports accurate memory for the distinctive feature itself, which may corroborate other evidence. An expert can describe the conditions under which attention is drawn away from a face; the expert cannot determine whether that happened to this witness.

This paper is for educational purposes only and is not legal advice. The author is a psychologist, not an attorney; summaries of law are general and must be verified against current authority or with a licensed attorney.

References

Fawcett, J. M., Russell, E. J., Peace, K. A., & Christie, J. (2013). Of guns and geese: A meta-analytic review of the ‘weapon focus’ literature. Psychology, Crime & Law, 19(1), 35–66. https://doi.org/10.1080/1068316X.2011.599325

Guynn, M. J., & Roediger, H. L., III. (1995). High-priority event instructions affect implicit and explicit memory tests. Psychological Research, 57(3–4), 192–202. https://doi.org/10.1007/BF00431280

Hope, L., & Wright, D. (2007). Beyond unusual? Examining the role of attention in the weapon focus effect. Applied Cognitive Psychology, 21(7), 951–961. https://doi.org/10.1002/acp.1307

Hunt, R. R. (1995). The subtlety of distinctiveness: What von Restorff really did. Psychonomic Bulletin & Review, 2(1), 105–112. https://doi.org/10.3758/BF03214414

Hunt, R. R., & McDaniel, M. A. (1993). The enigma of organization and distinctiveness. Journal of Memory and Language, 32(4), 421–445. https://doi.org/10.1006/jmla.1993.1023

Joint Service Committee on Military Justice. (2025). Amendments to the Military Rules of Evidence since 1984 (G. E. Maggs, Ed.; 2025 ed.). https://jsc.defense.gov/Portals/99/MRE%20Amendments%202025.pdf

Jones, A. R., Carlson, C. A., Lockamyeir, R. F., Hemby, J. A., Carlson, M. A., & Wooten, A. R. (2020). “All I remember is the black eye”: A distinctive facial feature harms eyewitness identification. Applied Cognitive Psychology, 34(6), 1379–1393. https://doi.org/10.1002/acp.3714

Kamp, S. M., Lenhof, C., Zeiler, D., Kaumanns, A., & Malmberg, K. J. (2026). The von Restorff effect in free recall, recognition, and source memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 52(6), 873–889. https://doi.org/10.1037/xlm0001518

Körner, H. M., Faul, F., & Nuthmann, A. (2024). Is a knife the same as a plunger? Comparing the attentional effects of weapons and non-threatening unusual objects in dynamic scenes. Cognitive Research: Principles and Implications, 9, Article 66. https://doi.org/10.1186/s41235-024-00579-1

Lane, S. M., & Houston, K. A. (2021). Understanding eyewitness memory: Theory and applications. New York University Press.

Light, L. L., Kayra-Stuart, F., & Hollander, S. (1979). Recognition memory for typical and unusual faces. Journal of Experimental Psychology: Human Learning and Memory, 5(3), 212–228. https://doi.org/10.1037/0278-7393.5.3.212

MacLeod, C. M. (2020). Zeigarnik and von Restorff: The memory effects and the stories behind them. Memory & Cognition, 48(6), 1073–1088. https://doi.org/10.3758/s13421-020-01033-5

Mather, M., & Sutherland, M. R. (2011). Arousal-biased competition in perception and memory. Perspectives on Psychological Science, 6(2), 114–133. https://doi.org/10.1177/1745691611400234

McDaniel, M. A., & Einstein, G. O. (1986). Bizarre imagery as an effective memory aid: The importance of distinctiveness. Journal of Experimental Psychology: Learning, Memory, and Cognition, 12(1), 54–65. https://doi.org/10.1037/0278-7393.12.1.54

Mitchell, K. J., Livosky, M., & Mather, M. (1998). The weapon focus effect revisited: The role of novelty. Legal and Criminological Psychology, 3(2), 287–303. https://doi.org/10.1111/j.2044-8333.1998.tb00367.x

Pickel, K. L. (1998). Unusualness and threat as possible causes of “weapon focus.” Memory, 6(3), 277–295. https://doi.org/10.1080/741942361

Pickel, K. L. (1999). The influence of context on the “weapon focus” effect. Law and Human Behavior, 23(3), 299–311. https://doi.org/10.1023/A:1022356431375

Schmidt, S. R. (1991). Can we have a distinctive theory of memory? Memory & Cognition, 19(6), 523–542. https://doi.org/10.3758/BF03197149

Schmidt, S. R. (2002). Outstanding memories: The positive and negative effects of nudes on memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 28(2), 353–361. https://doi.org/10.1037/0278-7393.28.2.353

Tulving, E. (1969). Retrograde amnesia in free recall. Science, 164(3875), 88–90. https://doi.org/10.1126/science.164.3875.88

Von Restorff, H. (1933). Über die Wirkung von Bereichsbildungen im Spurenfeld [The effects of field formation in the trace field]. Psychologische Forschung, 18(1), 299–342. https://doi.org/10.1007/BF02409636

Wallace, W. P. (1965). Review of the historical, empirical, and theoretical status of the von Restorff phenomenon. Psychological Bulletin, 63(6), 410–424. https://doi.org/10.1037/h0022001

Legal Authorities

Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993).

Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999).

State v. Chapple, 135 Ariz. 281, 660 P.2d 1208 (1983).

State v. Nordstrom, 200 Ariz. 229, 25 P.3d 717 (2001).

United States v. Houser, 36 M.J. 392 (C.M.A. 1993).

Fed. R. Evid. 702.

Mil. R. Evid. 702.

Mil. R. Evid. 1102.

Ariz. R. Evid. 702.

Ariz. Sup. Ct. Order R-23-0004 (amending Ariz. R. Evid. 702, effective Jan. 1, 2024).

Revised Arizona Jury Instructions (Criminal), Standard Criminal 45 (Identification) (6th ed. 2022).