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Adaptive memory Is survival processing special

Adaptive memory:Is survival processing special?

James S.Nairne *,Josefa N.S.Pandeirada

Department of Psychological Sciences,Purdue University,703Third Street,West Lafayette,IN 47907,USA

a r t i c l e i n f o Article history:

Received 2April 2008

revision received 5June 2008Available online xxxx Keywords:Evolution Memory Survival Recall

a b s t r a c t

Do the operating characteristics of memory continue to bear the imprints of ancestral selection pressures?Previous work in our laboratory has shown that human memory may be specially tuned to retain information processed in terms of its survival relevance.A few seconds of survival processing in an incidental learning context can produce recall levels greater than most,if not all,known encoding procedures.The current experiments further establish the power of survival processing by demonstrating survival processing advantages against an encoding procedure requiring a combination of individual-item and relational processing.Participants were asked to make either survival relevance deci-sions or pleasantness ratings about words in the same categorized list.Survival processing produced the best recall,despite the fact that pleasantness ratings of words in a catego-rized list has long been considered a ‘‘gold standard”for enhancing free recall.The results also help to rule out conventional interpretations of the survival advantage that appeal to enhanced relational or categorical processing.

Introduction

The capacity to remember,to recover the past in antic-ipation of the future,almost certainly evolved (Darwin,1859).Nature shaped the characteristics of our memory systems,primarily through natural selection,because ?t-ness advantages accrued as a consequence of memory’s operation (see Klein,Cosmides,Tooby,&Chance,2002;Tooby &Cosmides,1992).Yet,to what extent do the oper-ating characteristics of memory continue to bear the im-print of ancestral selection pressures?Are our memory systems ‘‘tuned”to achieve speci?c ends,particularly those related to survival and reproduction?Or,did memory evolve as an all-purpose machine,de?ned more by its ?ex-ibility than by its inherent constraints?

Our laboratory has maintained that human memory likely does contain functional specialization (see Barrett &Kurzban,2006).More speci?cally,we have suggested that memory is biased or tuned to remember ?tness-rel-evant information (Nairne &Pandeirada,2008;Nairne,Thompson,&Pandeirada,2007).It is unlikely that mem-ory evolved to be domain-general,or insensitive to con-tent,because not all events are equally important to remember.For example,it is usually more important to remember the location of a food source,or a predator,than it is to remember random events.This is not to sug-gest that our brains come pre-equipped with content-speci?c knowledge (e.g.,edible versus inedible plants),but rather that ?tness-relevant encodings are remem-bered particularly well.

In support of this proposal,Nairne et al.(2007)found that memory was signi?cantly enhanced relative to tradi-tional deep processing controls (Craik &Lockhart,1972)when random words were processed in terms of their rel-evance to a survival scenario.Participants were asked to imagine themselves stranded in the grasslands of a foreign land,without any basic survival materials,and to rate the relevance of words to ?nding steady supplies of food and water and protection from predators.Surprise free recall tests revealed an advantage for survival processing over a pleasantness rating task,typically considered to be one of the best deep encoding procedures (e.g.,Packman &Battig,1978),as well as over an alternative schematic control (moving to a foreign land)and to a condition requiring self-referential processing.More recently,we compared

*Corresponding author.Fax:+17654961264.

E-mail address:nairne@https://www.wendangku.net/doc/fc2742394.html, (J.S.Nairne).

Journal of Memory and Language xxx (2008)xxx–xxx

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survival processing to a host of deep processing controls—including forming a visual image,generation,and inten-tional learning—and survival processing produced the best recall(Nairne,Pandeirada,&Thompson,2008).

Mnemonic advantages for survival processing have now been demonstrated in other laboratories as well using alternative control scenarios(Kang,McDermott,&Cohen, in press;Weinstein,Bugg,&Roediger,2008).For example, survival processing produced better memory than a con-trol scenario involving the planning and execution of a bank heist(Kang et al.,in press).The bank heist scenario was chosen to match the novelty and potential excitement of the survival scenario,something that may have been lacking in the moving control scenario used by Nairne et al.(2007).Our laboratory has also found survival advan-tages compared to scenarios in which(a)people were asked to imagine themselves vacationing at a fancy resort with all of their needs taken care of,(b)eating dinner at a restaurant,and(c)planning a charity event with animals at the local zoo(Nairne&Pandeirada,2007;Nairne et al., 2007;Nairne et al.,2008).At this point,we believe,appeal-ing simply to the schema-like properties of the survival scenario,or to its coherence or novelty,is unlikely to ex-plain these advantages.

Instead,these experiments support the hypothesis that it is the?tness-relevance of the processing that is impor-tant to https://www.wendangku.net/doc/fc2742394.html,rmation encoded as a consequence of?tness-based processing is especially accessible and memorable—more memorable,in fact,than that produced by most(if not all)known encoding procedures,at least when free recall is used as the retention measure.At the same time,these experiments have revealed very little about the proximate mechanisms that actually produce the survival bene?t.Is survival processing special,arising from the action of some kind of special mnemonic adapta-tion,or can we explain the advantage using traditional explanatory tools?For example,one might claim that sur-vival processing is simply another form of‘‘deep process-ing”,albeit a particularly good one,leading to enhanced elaboration or distinctive encodings(see Hunt&Worthen, 2006).

Another possibility is that rating words for survival mimics a categorization task.Perhaps participants essen-tially encode the rated words into an‘‘ad hoc”category representing‘‘things that are relevant in a survival situa-tion.”Once primed by the rating task,the category struc-ture could support an accessible and ef?cient retrieval plan(Tulving&Pearlstone,1966).Put more generally, one can conceive of the survival rating task as inducing a form of relational processing.As people rate the items, they process ostensibly unrelated words along a common dimension of similarity—relevance to a survival context. It is well-established that relational processing of unre-lated items leads to improved free recall,partly because the encoded dimension of similarity helps to restrict the set of possible recallable items at the point of test(Hunt &McDaniel,1993;Nairne,2006).Note this is a completely conventional account of the survival advantage:Survival ratings induce people to encode target items into a cate-gorical structure that is particularly accessible during retrieval.

Such an account generates an obvious prediction:If the to-be-rated words are inherently related(e.g.,if the list is categorized)then any relational processing induced by the survival rating task should be less useful to retention (see Burns,2006;Mulligan,2006).A number of studies have found that relational processing of items in a related list,such as sorting items from an obviously categorized list into categories,produces no particular mnemonic advantages,at least when compared to identical process-ing of words in an unrelated list(e.g.,Burns,1993;Einstein &Hunt,1980;Hunt&Einstein,1981).The category struc-ture inherent in the list affords a suf?cient retrieval‘‘plan”for use in recall(e.g.,the list contained pieces of furniture, weapons,and so on)so further relational processing is redundant(although see Engelkamp,Biegelmann,& McDaniel,1998).In fact,encoding procedures that draw attention to the unique characteristics of the to-be-recalled items(such as rating items for pleasantness or familiarity) promote the best recall when lists are categorized.The list structure enables one to restrict the target search set effec-tively,and the individual-item processing helps one dis-criminate items within the search set that did or did not actually occur on the memory list(see Nairne,2006). Researchers have been able to explain a variety of mne-monic phenomena by appealing to trade-offs between item-speci?c and relational processing(e.g.,Hunt&Seta, 1984;Klein,Kihlstrom,Loftus,&Aseron,1989;Klein& Loftus,1988;Mulligan,1999;Mulligan,2001);it is certainly possible that similar logic can be used to explain the advantages seen after survival processing.

The current experiments were designed to test these ideas,as well as to compare the mnemonic value of sur-vival processing against yet another powerful encoding procedure:Individual item processing of words presented in a categorized list.In all three experiments,participants were asked to make rating decisions about words in a cat-egorized list prior to a surprise free recall test.Experiment 1used a between-subject design to compare the effects of survival processing to a prototypical individual-item pro-cessing task—rating items for pleasantness.Experiment2 replicated the results of Experiment1using a within-sub-ject design.Finally,in Experiment3a non-?tness-relevant scenario,vacationing at a fancy resort in a foreign land, was used instead of the survival scenario.

Experiment1

In Experiment1,participants were required to make survival relevance decisions about items in an obviously categorized list.To enhance the salience of the category structure,we blocked the category items during presenta-tion and used categories that seemed inherently survival-related(animals,fruits,vegetables,and human dwellings). In a separate control condition,participants were asked to make pleasantness ratings about exactly the same items prior to the surprise recall test.Again,individual-item pro-cessing of items in a categorized list is generally thought to be the best procedure for maximizing free recall(Hunt& McDaniel,1993).If survival processing simply induces a form of relational processing—e.g.,?tting words into an ad hoc category of survival relevance—then we would

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expect the typical survival advantage to disappear or even reverse in the current experiment.

Method

Participants and apparatus

Eighty Purdue undergraduates participated in exchange for partial credit in an introductory psychology course.Par-ticipants were tested individually in sessions lasting approximately30min.Stimuli were presented and con-trolled by personal computers.

Materials and design

Stimulus materials were drawn from the updated Battig and Montigue norms(Van Overschelde,Rawson,& Dunlosky,2004)and consisted of eight exemplars from four unique categories:Four-footed animals,fruits,vegeta-bles,and a type of human dwelling.The32words were presented blocked by category(e.g.,all eight animals were presented together)but exemplar order within each cate-gory was randomly determined.Four different orders of category presentation were created to ensure that each category occurred equally often in each list quartile across participants.Order of exemplar presentation within each category was the same in all versions.Two items from an additional two categories(a natural earth formation;an article of clothing)were selected to use as practice items.

A simple between-subject design was used:Partici-pants in each group were asked to rate the same words, presented in the same random orderings,in one of the two rating scenarios(N=40in each group).The rating task was followed immediately by a short digit recall task prior to a?nal unexpected free recall task.Except for the rating scenario,all aspects of the design,including timing,were held constant across participants.

Procedure

On arrival in the laboratory,participants were randomly assigned to one of the two rating scenarios with the follow-ing instructions:

Survival.In this task we would like you to imagine that you are stranded in the grasslands of a foreign land,with-out any basic survival materials.Over the next few months, you’ll need to?nd steady supplies of food and water and protect yourself from predators.We are going to show you a list of words,and we would like you to rate how rel-evant each of these words would be for you in this survival situation.Some of the words may be relevant and others may not—it’s up to you to decide.

Pleasantness.In this task,we are going to show you a list of words,and we would like you to rate the pleasantness of each word.Some of the words may be pleasant and others may not—it’s up to you to decide.

Stimuli were presented individually,centered on the screen,for?ve s apiece and participants were asked to rate the words on a?ve-point scale,with one indicating totally irrelevant(unpleasant)and?ve signifying extremely rele-vant(pleasant).The rating responses,one through?ve, were displayed just below the presented stimulus and par-ticipants responded by selecting the button that corre-sponded to the rating of their choice.Everyone was cautioned to respond within the?ve s presentation win-dow and no mention was made of a later retention test.

A short practice session,containing four to-be-rated words, preceded the actual rating task.

After the last word was rated,instructions appeared for the digit recall task.For this task,seven digits,ranging be-tween zero and nine,were presented sequentially for one s apiece and participants were required to recall the digits in order by typing responses into a text box.The digit recall task proceeded for approximately two min.Recall instruc-tions then appeared.Participants were instructed to write down the earlier-rated words,in any order,on a response sheet.The?nal recall phase proceeded for10min and par-ticipants were asked to draw a line on the recall sheet,un-der the last recalled word,after each min of recall.A clock was displayed on the computer monitor and a‘‘beep”sounded every min signaling the participants to draw the line.

Results and discussion

Cumulative recall functions for the survival and pleas-antness groups are shown in Fig.1.During the?rst two mins of recall the groups performed similarly,but a reli-able survival advantage emerged by the?ve min mark [F(1,78)=5.207,MSE=12.10,p<.03].During the second half of the recall period,on average,very few additional items were recalled(survival=1.6;pleasantness=1.8);a separate analysis on the number of item gains between?ve and10mins revealed no signi?cant difference between groups[F(1,78)<1.0].Overall,for the entire10min recall period,a survival advantage was present(survival=.67; pleasantness=.62)but only marginally signi?cant using a two-tailed test[F(1,78)=3.70,MSE=.012,p<.06].1 The top half of Table1presents category clustering data for recall using the adjusted ratio of clustering,or ARC score(Roenker,Thompson,&Brown,1971).The ARC score measures the extent to which members of the same cate-gory tend to be recalled together and is often used as a measure of relational processing(see Burns,2006).An ARC score of1.00indicates perfect clustering and a score of zero indicates chance-level clustering.As expected,the ARC scores were well above zero,but no signi?cant differ-ences were found between the survival and pleasantness conditions[F(1,78)=1.87,MSE=.04.p>.10].

Table1reports intrusion data as well,or the extent to which participants recalled items that were not on the list. The mean number of intrusions was low for both groups, but signi?cantly more intrusions occurred in the survival group[F(1,78)=15.31,MSE=1.51,p<.001].Thus,

1The fact that the survival advantage was highly signi?cant after?ve-mins and only marginally signi?cant after10mins might have been due,in part,to the fact that participants tended to recall slightly more new items in the pleasantness condition during the second half of the recall period. Such a pattern may signify relatively more individual-item processing in the pleasantness condition because steady increases in the recovery of new items(item gains)can be one hallmark of individual-item processing. However,the difference in item gains was not statistically signi?cant between groups and assumptions about individual-item processing and item gains may depend importantly on whether list items are related(see Burns,1993).

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although there is no evidence for differential relational processing between the groups,it is possible that survival processing effectively lowered the threshold for response output(for unknown reasons).In a follow-up analysis we examined performance exclusively for participants who failed to commit an intrusion.A survival advantage was still detected in overall proportion correct recall(Sur-vival=0.69;Pleasantness=0.63),but the difference was signi?cant only in a one-tailed t-test[t(56)=1.87].

The bottom half of Table1shows the mean rating and response time data for each group.The average rating was slightly higher when items were processed for survival relevance,but the difference did not approach statistical signi?cance[F(1,78)=1.67,MSE=.21].Similarly,people took slightly longer to decide about survival relevance, compared to a pleasantness decision,but the difference once again was not statistically signi?cant[F(1,78)=1.94, MSE=148230.8].Previous work in our laboratory has shown that neither average ratings nor response times are capable of explaining survival processing advantages in recall(e.g.,Nairne et al.,2007);a similar conclusion seems appropriate here.

Overall,then,the results of Experiment1provide an-other demonstration of the power of?tness-relevant pro-cessing.Processing items for their survival relevance produced better retention than a standard deep processing control(rating items for pleasantness).More importantly, the present control condition—individual item processing in a categorized list—is thought generally to maximize free recall because it affords both relational and distinctive information to encoding(Hunt&McDaniel,1993).Previ-ous work in our laboratory has shown that a few seconds of survival processing produces recall levels that exceed the‘‘best of the best”of the known encoding procedures, including the formation of a visual image,generation, and intentional learning(Nairne et al.,2008).Our conclu-sion,that survival processing is one of the best—if not the best—encoding procedures yet identi?ed in human memory research,receives further support from the pres-ent experiment.

The results of Experiment1also help to rule out one possible interpretation of the survival advantage.As dis-cussed earlier,processing items for their survival relevance may induce a form of relational processing.When unre-lated items are processed in terms of a common theme (survival relevance),it is conceivable that participants en-code similarities among the items,or?t the items into an accessible category structure.In the present case,the list possessed a salient category structure,and the categories were selected to be survival-relevant,so further relational processing should have been redundant and of little addi-tional use in recall.A survival advantage was still obtained, and the measure of category clustering suggested equiva-lent amounts of relational processing across the groups. Experiment2

Experiment2was designed to replicate the results of Experiment1using a within-subject design.Once again, participants received a list of blocked category items,but on a random half of the trials they were required to make either a pleasantness or a survival rating.Within each cat-egory half of the items were rated for survival and the other half for pleasantness.The rating task was followed by a surprise free recall test for the rated items.

The use of a within-subject design in this context is important for two reasons.First,because people are mak-ing both survival and pleasantness decisions about items in exactly the same categories,any enhancing effect that either form of processing might have on category accessi-bility should bene?t items equally in both conditions.Sec-ond,the intrusion data in Experiment1suggested that survival processing may have lowered the response output threshold relative to the pleasantness condition.In the present experiment,because of the within-subject design, any such tendency would be expected to affect the recall of items in both the survival and pleasantness conditions.

Table1

Averages and standard deviation of the ARC scores,number of intrusions,

rating and response time

Survival Pleasantness

M SD M SD

Clustering0.470.210.530.18

Intrusions 1.58 1.450.500.961

Rating 3.210.48 3.070.44

Response time(ms)2274.4449.42154.4307.4

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Method

Participants and apparatus

Thirty-two Purdue undergraduates participated in ex-change for partial credit in an introductory psychology course.Participants were tested individually in sessions lasting approximately30min.Stimuli were presented and controlled by personal computers.

Materials and design

As in Experiment1,stimulus materials were drawn from the updated Battig and Montigue norms (Van Overschelde et al.,2004)and consisted of eight exemplars from four unique survival-relevant categories: Four-footed animals,weather phenomenon,vegetables, and a type of human dwelling.The32words were presented blocked by category and exemplar order within each category was randomly determined.Four different orders of category presentation were created to ensure that each category occurred equally often in each list quartile across participants.Order of exemplar presenta-tion within each category was the same in all versions.

Two additional words from each category were selected to be used as practice items.

A within-subject design was employed:For a random half of the items in each blocked category,participants were instructed to make either a survival or a pleasantness rating decision.Task order was also counterbalanced across participants to ensure that each word was rated equally often for survival and pleasantness.

Procedure

On arrival in the laboratory,participants were told they would be asked to rate words in two ways.For some words they would provide a pleasantness rating;for other words, they would rate the word’s relevance to a survival situa-tion.General rating instructions were provided and were identical to those used in Experiment1.A short practice session preceded the main rating session.

Stimuli were presented individually for?ve s in the cen-ter of the screen.Above each word a question was pre-sented specifying the rating decision for that word(‘‘How PLEASANT is this word?”,or‘‘How relevant is this word to the SURVIVAL situation?”).Below each word,the rating scale was presented(ranging from one to?ve)along with the relevant labels.Participants responded by selecting the button that corresponded to the rating of their choice. The rating tasks were distributed randomly with the con-straint that no more than two words were rated on the same dimension in a row.After the rating session,a short distractor task preceded initiation of the surprise free re-call test.Both the distractor and recall task were the same as presented in Experiment1.

Results and discussion

The data of main interest are presented in Fig.2,which shows proportion correct recall for words rated for survival and pleasantness.Replicating Experiment1,recall in the survival condition was higher than in the pleasantness condition[F(1,31)=4.48,MSE=5.58,p<.05].Out of the 32participants,22recalled more survival items,nine re-called more items rated for pleasantness,and there was one tie.Because of the within-subject design,with both conditions represented equally in all categories,neither the ARC scores nor the overall intrusion rates are presented.

In Experiment1,mean relevance ratings and response times favored the survival task,but neither was signi?cant in the statistical analyses.In Experiment2,however,the mean relevance ratings for survival were signi?cantly higher than the ratings for pleasantness(3.6vs. 2.8; F(1,31)=45.2,MSE=.23);survival ratings also took signif-icantly longer to complete(2950.3vs.2805.3; F(1,31)=4.33,MSE=77754.1).Thus,it is conceivable that the survival recall advantage in Experiment2is partly attributable to either a congruity effect(i.e.,recall is a po-sitive function of the match between the encoding context and the item;Schulman,1974),or to the fact that survival relevance decisions are more effortful.We think both of these interpretations are unlikely,based on previous work showing strong survival recall advantages without corre-sponding differences in average ratings or response times (see Experiment1;also,Nairne et al.,2007).

However,as a further check we performed several addi-tional item-based analyses.First,we calculated the recall and rating data for each item when it was processed for survival or pleasantness.We then looked at the correlation between recall and rating,which was nonsigni?cant in both cases(Pearson r for survival was0.19;r for pleasant-ness was0.28).Next,we correlated the size of the survival effect(the difference in recall when the item was rated for survival versus pleasantness)with the size of the rating difference(the difference in rating when the word was rated for survival versus pleasantness);this correlation was small and nonsigni?cant(Pearson r=.09).We also performed a median split on the rating differences and looked at the size of the survival effect only for those items that were in the lower half(i.e.,those items showing small or no survival rating advantages).For these16items,the

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average rating difference between survival and pleasant-ness was a nonsigni?cant0.20(Survival=3.4;Pleasant-ness=3.2),yet the overall survival advantage remained in recall(Survival=.66;Pleasantness=.55;t(15)=2.80, p<.02).

We performed the same item analyses for the response times.Again,there was a nonsigni?cant correlation be-tween the task-based response times and the survival ef-fect in recall(Pearson r=à.23).We also once again looked at the16items that showed the smallest(or no) survival response time advantages.For these items,aver-age response times were actually longer for the pleasant-ness task(Survival=2738.9;Pleasantness=2932.0),yet the survival advantage remained in recall(Survival=.73; Pleasantness=.61;t(15)=2.58,p<.03).Consequently,as in our earlier reports,differences in average ratings or re-sponse times cannot fully explain the survival advantages seen in free recall(Nairne et al.,2007).

With respect to free recall,the results of Experiment2 replicate those of Experiment1.A reliable survival advan-tage was obtained,even though the target list was catego-rized and the control condition was designed to engage a combination of individual item and relational processing. The list structure should have minimized any mnemonic effect of relational processing,yet a survival advantage still emerged.The survival bene?t is even more convincing in Experiment2because a within-subject design was em-ployed.To the extent that survival processing simply in-creased accessibility of the list categories,or lowered the response output threshold,then related performance ef-fects should have occurred for both survival and pleasant-ness items.

Experiment3

In the existing literature,it is extremely dif?cult to?nd examples of encoding procedures that produce superior free recall to a condition requiring pleasantness ratings in a categorized list.The results of the?rst two experiments, as a consequence,strongly establish the power of?tness-relevant processing compared to traditional encoding tasks.Yet,the survival rating procedure used in our exper-iments is unusual in the sense that processing occurs with-in the context of a relatively rich and cohesive cover scenario(survival in the grasslands of a foreign land). Although we have compared the survival scenario to other control scenarios,such as moving to a foreign land,it is possible that any schema-based encoding produces better recall than individual-item processing in a categorized list. Experiment3was designed to examine this possibility.

Experiment3was an exact replication of Experiment2, except that the survival scenario was replaced with a con-ceptually rich,but non-?tness-relevant,alternative sce-nario.Participants were asked to rate the relevance of words to a vacation scenario(see also Nairne et al., 2008).If schema-based processing of words in a catego-rized list is suf?cient to induce superior retention,then we expect to replicate the pattern seen in Experiment2—a recall advantage for the scenario condition over the pleasantness condition.On the other hand,if the important dimension is the?tness-relevance of the processing,then we anticipate the more traditional result—the best recall for a condition requiring individual-item processing in a categorized list.

Method

Participants and apparatus

Materials and design

The materials and design employed in Experiment2 were used in Experiment3,with the exception that the vacation scenario was substituted for the survival scenario.

Procedure

All aspects of the procedure mimicked those of Experi-ment2except for the use of the following vacation scenario:

Vacation.In this task,we would like you to imagine that you are enjoying an extended vacation at a fancy resort in the grasslands of a foreign land.All your basic needs are ta-ken care of but,over the next few months,you’ll want to investigate your surroundings and?nd different activities to pass the time and maximize your enjoyment.We are going to show you a list of words,and we would like you to rate how relevant each of these words would be for you in this vacation situation.Some of the words may be relevant and others may not—it’s up to you to decide.

Results and discussion

The?nal free recall results are shown in Fig.3,pre-sented as a function of encoding condition.An ANOVA re-

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vealed a signi?cant effect of condition[F(1,31)=4.25, MSE=.02,p<.05].Importantly,however,in this case rat-ing items for pleasantness produced the best recall,revers-ing the pattern found in Experiment 2.For the32 participants,12recalled more items rated under the vaca-tion scenario,17recalled more items rated for pleasant-ness,and there were three ties.

Analysis of the rating data revealed no signi?cant differ-ences between the two encoding conditions(Vaca-tion=2.6,Pleasantness=2.6;F(1,31)<1.0).Note that the average relevance ratings for the vacation condition, although identical to the ratings given for pleasantness, were lower than those given for the survival condition in Experiment 2.Given that the list was categorized,and the categories were selected to be survival-related,this ?nding is not particularly surprising.We have directly compared survival processing with a comparable‘‘vaca-tion”control in previous work,using a list of random words,and a signi?cant survival advantage was obtained with little or no differences in average ratings(Nairne et al.,2008).It is also interesting to note that the recall lev-els for the words rated for pleasantness were higher than those seen in Experiment2,even though exactly the same design and materials were used.We have no explanation for this?nding,although the pattern across experiments could be interpreted as an example of a list strength effect (Tulving&Hastie,1972).However,the survival advantage seen in Experiment2was also seen in Experiment1,which used a between-subject design,so one cannot account for the effect overall by appealing to encoding variable inter-actions(e.g.,list strength effects,differential output inter-ference,etc.).

For the response time data,participants took signi?-cantly longer to generate a rating in the vacation condition [Vacation=2907.1,Pleasantness=2658.8;F(1,31)=18.84, MSE=52333.6,p<.001].This?nding is important because it further dissociates response time and recall perfor-mance.In this case,the more‘‘effortful”encoding task—processing the relevance of items to a vacation scenario—was associated with poorer free recall performance.As noted throughout,it is unlikely that any simple appeal to rating or response time differences will be able to explain the encoding-based differences in recall seen in these experiments.

Overall,the results of Experiment3conformed to the expectations of conventional memory theory.It is widely acknowledged that free recall performance bene?ts greatly from encoding procedures that induce a combina-tion of individual item and relational processing.Both are necessary in order to(a)restrict the memory search set and(b)discriminate effectively among items within the set that either did or did not occur(e.g.,Hunt& McDaniel,1993;Nairne,2006).In the present case,rat-ing items for pleasantness in an obviously categorized list led to the best recall,compared to a‘‘scenario”con-dition that presumably induced primarily relational pro-cessing.As a result,the data lower the chances that the survival advantages seen in Experiments1and2can be attributed simply to the use of scenario-or schema-based processing.General discussion

The crux of the functionalist agenda is the recognition that memory is functionally designed(Nairne,2005; Sherry&Schacter,1987).As a product of evolution,we can assume that our ability to remember has been sculpted by natural selection to achieve speci?c ends,much like the heart is functionally designed to pump blood,or the kidneys to?lter impurities(Klein et al.,2002;Nairne& Pandeirada,2008).Although few question the adaptive value of memory,or that the ability to remember arose as a product of evolution,whether human memory contin-ues to bear the footprints of ancestral selection pressures remains an open,and ultimately empirical,question.

Our laboratory has suggested that memory may be tuned to process and retain?tness-relevant information—that is,information that is relevant to survival and ulti-mately to reproduction.Not all information is equally important from a?tness perspective,so it is reasonable to assume that our memory systems show domain-speci-?city,or sensitivity to information content.In empirical support of this idea,we have found that processing infor-mation in terms of its survival relevance leads to particu-larly good retention(Nairne et al.,2008)—better retention,in fact,than most,if not all,known encoding techniques(e.g.,imagery,self-reference,generation,inten-tional learning),at least when free recall is used as the retention measure.

The current experiments were designed with two pri-mary goals in mind.First,we were interested in comparing the effectiveness of survival processing to yet another powerful encoding technique,one that encourages individ-ual-item processing of words in a categorized list.Lots of studies have found that item-based encoding tasks,such as rating an item for pleasantness,lead to especially good recall when a list is categorized because such processing yields distinctive encodings in a context of similarity(Hunt &McDaniel,1993;Hunt&Smith,1996).Despite the recog-nized superiority of such an encoding procedure,a few sec-onds of survival processing produced higher levels of free recall in both Experiments1and2of the present report.

Second,we were interested in testing one possible,and conventional,interpretation of the survival advantage.Pro-cessing items in terms of their survival relevance can be conceived as a form of relational processing.Items are pro-cessed with respect to a single context,surviving in the grasslands,and it is possible that participants encode the items into a relevant ad hoc category that is particularly accessible at retrieval.In the present experiments,how-ever,survival processing of words in a categorized list, one containing blocked and salient survival-related catego-ries,continued to produce a reliable survival advantage compared to pleasantness processing.In such a context, any survival-based categorical processing should have been redundant with the information afforded by the list structure and therefore of limited usefulness in recall.

The survival advantage was obtained as well in a with-in-subject design,one in which participants made both survival and pleasantness decisions about items in exactly the same categories.The survival advantage is important in

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this context because any effect that survival processing might have had on category salience or accessibility should have contributed to recall of both the survival and pleas-antness-rated items.The data of both Experiments1and 2,as a result,signi?cantly lower the chances that enhanced relational(or categorical)processing is responsible for the demonstrated survival advantage.Moreover,in Experi-ment3participants were asked to rate the relevance of words to a vacation scenario,one that presumably also in-duced relatively greater amounts of relational processing than the pleasantness task,yet no recall advantage was ob-tained.Instead,the more typical result was obtained—en-hanced memory for the pleasantness rating task, presumably because it led to combined individual-item and relational processing.

Do the results of these and other experiments indicate that survival processing is special?Certainly from an empirical perspective survival processing is a powerful encoding technique,leading to better free recall than a variety of standard encoding techniques.In addition,the survival advantage is easily obtained in both within-and between-subject designs,unlike a number of other encod-ing-based mnemonic phenomena(e.g.,the generation ef-fect,word frequency effect,the effect of bizarre imagery, emotionality,etc.).The typical interpretation of such ef-fects is that the encoding task leads people to focus atten-tion on the individual attributes of the item at the expense of noting relationships among the items(e.g.,as indexed by poorer memory for temporal order;see McDaniel& Bugg,2008,for a review).We have yet to examine the ef-fect of survival processing on the retention of temporal or-der,but the current experiments revealed no differential relational processing between conditions.Clearly,any det-rimental effects that survival processing may have on or-der memory are compensated by other mnemonic advantages.

Perhaps the simplest‘‘explanation”of the survival advantage is that it leads to more item-based elaboration, or‘‘spread of processing”(e.g.,Craik&Tulving,1975).Sim-ilar appeals are commonly used to explain the advantages of semantic or deep processing—i.e.,processing the mean-ing of an item leads to more connections between the tar-get item and other information in memory.More elaboration,in turn,increases the chances that appropriate retrieval cues can be accessed in free recall environments. However,there is no obvious reason to expect survival pro-cessing,which is a relatively novel task,to produce more elaboration than other procedures that have more rele-vance to everyday experiences(e.g.,moving,restaurant, or vacation scenarios).One of the failings of modern mem-ory theory is the absence of easily—applied and indepen-dent indices of elaboration,so‘‘elaboration”accounts remain speculative and dif?cult to test at this point.

Regardless of the proximate mechanism,it is clearly adaptive for people to retain?tness-relevant material. Selection advantages could easily have accrued from a memory system tuned to remember the location of food or predators,although it is exceedingly dif?cult to estab-lish the existence of true cognitive adaptations(Andrews, Gangestad,&Matthews,2002;Williams,1996).Whether humans evolved functionally specialized circuitry for remembering?tness-relevant material,or whether reten-tion advantages for?tness-relevant processing arise as a by-product of other more general mnemonic mechanisms, remains unclear.At the same time,it is na?ve to assume that our memory systems lack functional design.The capacity to remember evolved because of the selection advantages it provided;consequently,we should antici-pate that memory contains features that are selectively tuned to solving adaptive problems related to?tness(Klein et al.,2002;Nairne,2005;Rozin,1976).

Still,one might not expect recall advantages of the type demonstrated in the current experiments for all kinds of ?tness-relevant materials.Free recall requires a search en-gine,or retrieval process,that accesses stored information using a criterion of recent occurrence.It is an episodic task, one that requires people to generate items that occurred at a speci?ed time,in a speci?ed location,as de?ned by the experimental context(Nairne,1991;Tulving,1983).From a functional perspective,the need to remember some?t-ness-relevant material,such as the location of food,water, or a possible predator,is often likely to be time-dependent. In fact,Anderson and Schooler(1991)have shown that for-getting functions often mimic the way that events occur and recur temporally in the environment.Other kinds of ?tness-relevant information,such as social and personal characteristics(e.g.,is that person a cheater?),may not show the same kinds of sensitivities to occurrence(see Barclay&Lalumière,2006;Mehl&Buchner,2007). Conclusion

Our laboratory has made the strong claim that survival processing represents one of the best—if not the best—encoding procedures yet discovered in the memory?eld (Nairne et al.,2008).Is such a claim justi?ed?Empirically, we believe the case is strong:Not only does a simple sur-vival rating produce better free recall than a veritable ‘‘who’s who”of known deep encoding procedures,but it also produces better memory than the prototypical combi-nation of individual-item and relational processing(pleas-antness ratings in a categorized list).As noted above, survival processing is one of the few techniques ever shown to produce higher levels of recall than a task requir-ing pleasantness ratings in a categorized list.Regardless of one’s ultimate interpretation of the survival advantage, then,adopting a functional perspective can lead to novel empirical?ndings that may challenge existing perspec-tives on retention.

Acknowledgment

Josefa N.S.Pandeirada was supported by fellowship SFRH/BPD/21534/2005from the Portuguese Foundation for Science and Technology.

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