OPINION
FABE, Chief Justice.I. INTRODUCTION
When Chad Paul and Sharon McLane divoreed in 2006, they entered into a court-approved settlement agreement granting Sharon physical custody of their six-year-old daughter Alexis during the school year in Alaska and Chad summer visitation in IIli-nois. While Alexis was with Chad for the *1041summer visitation, Chad suggested to Sharon that they reverse their custody arrangement. The parties discussed this possibility through the end of the summer, but Sharon refused to formalize a custody change and insisted that Alexis be returned to her. Chad then filed a motion for modification of custody and Sharon filed a motion for enforcement of the settlement agreement. The superior court granted Chad's motion for modification, awarding him school-year custody and granting Sharon summer visitation. Sharon appeals. Because a modification of custody requires a substantial change in cireum-stances, and because no substantial change of cireumstances was demonstrated here, we reverse the modification of custody awarding Chad custody during the school year.
II. FACTS AND PROCEEDINGS
Chad Paul and Sharon McLane were married on February 14, 1999 and divorced on June 20, 2006. Their daughter, Alexis Rose, was born on November 18, 2000. Before the divorce was final, Chad and Sharon signed a settlement agreement drafted by Sharon's counsel. Among other things, the settlement agreement divided the marital property and provided for spousal support. The settlement agreement also incorporated a parenting plan through which the parties agreed to share legal custody of Alexis, providing Sharon with primary physical custody and giving Chad scheduled summer and holiday visitation. The parenting plan was drafted by Chad without the aid of legal representation.
The settlement agreement required that custody modifications would be in writing, signed by both parties, or by court order. The parenting plan allowed flexibility "for special events, changes in cireumstances and to adjust to Alexig'[s] schedule" if both parties agreed, but provided that the parties' agreement remained in effect when the parties did not agree to a change.
Shortly after he signed the settlement agreement, Chad moved to Illinois. The parties had anticipated this move and included provisions in the settlement agreement that would take effect when Chad and Sharon lived in different communities. The settlement agreement and parenting plan were incorporated into the divorce decree on June 20, 2006. In early 2007 Chad remarried; his new wife had two young children from a previous relationship.
In June 2007, while Alexis was with Chad in Illinois for summer visitation, Chad suggested to Sharon that they reverse their agreement to give Chad school-year custody. Discussions on this topic took place throughout the summer. In late July Sharon sent Chad $2,000 that she described as child support for Alexis, along with some school clothes. Chad drafted a revised parenting plan to reverse the custody arrangement and mailed it to Sharon. But Sharon never signed the paperwork modifying custody.
On August 20, 2007, Alexis started first grade in Illinois. Two days later, Sharon called Alexis to ask how school was going. According to Sharon, Alexis said she wanted to come back to Alaska. Sharon then heard Chad talking to Alexis in the background, and Alexis told her mother, "I got to go, Daddy's mad." Sharon concluded that Chad had been monitoring her phone calls with Alexis, thwarting Alexis's attempts to tell Sharon that she wanted to return to Alaska. Soon afterward, Sharon told Chad that she would not agree to modify the custody arrangement and insisted Alexis be returned to Alaska.
Chad filed a motion to modify custody on August 27, arguing that the alleged informal agreement to allow Alexis to remain in Illinois constituted a substantial change of cireumstances. Chad offered his e-mail correspondence with Sharon as proof of the alleged informal agreement. The same day, Sharon filed a motion to enforce the existing custody agreement and to return Alexis to Alaska. She argued that Alexis should have been returned on July 29, and that despite her willingness to consider a different arrangement, the existing agreement remained in effect because no modification had been agreed to in writing, as the settlement agreement required. In her opposition to Chad's motion for modification of custody, she also contended that her e-mails with Chad were protected settlement negotiations and inadmissible under Alaska Rule of Evidence 408.
*1042The motions were considered on an expedited basis, and on August 29 the superior court held a hearing at which Sharon and Chad were the only witnesses. After a short recess, the superior court issued its decision from the bench, finding that Sharon and Chad had agreed that Alexis would remain in Illinois for the school year, although the agreement had not been placed in writing or formalized and Sharon had later changed her mind.
The superior court then analyzed Alexis's best interests, concluding that the parties were "even-steven" on the factors listed in the statute and that they were equally capable and loving parents. It nonetheless granted the modification of custody in Chad's favor, stating:
I believe and I find that a decision was made for Alexis to stay and that Sharon thought differently and changed her mind late. Now, that's usually what a writing is for. It's so that people actually have a written signature that helps them realize, this is the point of no return....
In this case, though, I believe that the point of not returning-the decision point was reached earlier and that was when Alexis was not returned[,] when school clothes were sent, money was sent, and that the decision was made.
And based upon this finding ... and the child's expectations at this point ... it is appropriate to modify the custody ... so that Alexis stays with Mr. Paul for this school year with the visitation as the parties have agreed.
The superior court went on to clarify that the modification was temporary, ruling that Alexis was to stay with Chad for this school year but that future custody was open to discussion: "[It is appropriate to modify the custody ... so that Alexis stays with Mr. Paul for this school year.... The parties need to discuss the future with either a child development counselor [or] school counsel-orgs...." When Sharon's counsel asked whether there could be a modification of custody at the end of the year, the superior court responded: "Yeah, and I hope the parties can sort [it] out by agreement, but we'll do it by a motion, if not, ... this is for one-for the next year or for the school year." And in response to whether the modification of custody switched the prior arrangement, the superior court responded, "Yes. But based upon the testimony, the parties were going to see how it went and then they'll reconsider what they're going to do." The parties agree that the superior court's modification of custody was to last only through the 2007-2008 school year, and during oral argument before this court, they noted the superior court's recent sua sponte appointment of a child eustody investigator to reevaluate custody at the end of this temporary period.
Sharon filed a motion for reconsideration on September 10, in which she raised a number of arguments, including (1) that there was no written agreement to change custody as the settlement agreement required; (2) that there had been no change in cireum-stances to support a modification; (8) that the settlement agreement should have been enforced; and (4) that a custody investigator should have been appointed. Three days later, the superior court denied the motion for reconsideration. Sharon appeals the custody modification in favor of Chad.
III. STANDARD OF REVIEW
We review a trial court's child custody modification decision deferentially, reversing the decision only when the lower court abused its discretion or when its controlling findings of fact were clearly erroneous.1 Abuse of discretion is established if the lower court "considered improper factors in making its custody determination, failed to consider statutorily mandated factors, or assigned disproportionate weight to particular factors while ignoring others."2 We conclude that a finding of fact is clearly erroneous if, after reviewing the record as a whole, we are left with a definite and firm conviction that a mistake has been made.3
*1043IV. DISCUSSION
A. The Superior Court Erred in Granting the Motion To Modify Custody Because There Had Been No Substantial Change in Circumstances.
Under AS 25.20.110(a), an award of custody "may be modified if the court determines a change in cireumstances requires the modification ... and the modification is in the best interests of the child." The movant must prove a substantial change in cireum-stances as a threshold matter.4 We require this showing to maintain continuity of care and to avoid disturbing and upsetting the child with repeated custody changes.5 We have cautioned that "[clhildren should not be shuttled back and forth between divorced parents unless there are important cireum-stances justifying such change."6
Here, the superior court concluded that the informal agreement between Chad and Sharon constituted a substantial change in cireumstances and that modifying custody would serve Alexis's best interests. Sharon agrees that she and Chad had discussed and experimented with the possibility of changing their custody arrangement but she maintains that no agreement was ever finalized and that the experimental arrangement lasted no more than three weeks.7 Therefore, she argues, the requisite substantial change in circumstances was not established.
In Morino v. Swayman, we recognized that "[clustodial parents should have the flexibility to experiment with new visitation schedules without fearing that every temporary change could be the basis for modifying visitation.8 We concluded that a change lasting ten months could warrant a modification in visitation,9 but expressly noted that "[olf course, experimental changes lasting only a few months should not qualify as a change in circumstances."10 We warned that "if temporary variations in visitation schedules always constituted a substantial change in cirenmstances, primary custodians would be discouraged from allowing any favorable deviation from the visitation order."11 It is important to allow parents leeway to cooperate and experiment with custody. Characterizing a short-term temporary and informal custody arrangement as a substantial change in circumstances could "discourage parents from being generous with each other in custody matters and, to that extent, run[ ] counter to the goals of Alaska's family law and the needs of Alaska's children of divorce.12
In this case, it is undisputed that the informal arrangement lasted only approximately three weeks.13 In Morino, we emphasized that "the change in circumstances required to modify visitation ... is not as great as that required for a change in custody."14 Here, modification of primary physical custody is at issue, setting an even higher bar to prove a change in circumstances.
Chad concedes that the duration of the change "was not lengthy," but he claims the distinguishing factor is that "there was an gctual agreement to modify custody" here. The record nonetheless indicates that the informal agreement was not only short-lived but experimental. As Chad testified, "we had discussed that we were going to have her go to school here for the year [in] Illinois, then we would readdress it next year to see how she was doing and we'd rediseuss custody next year also, see what was in her best interest." Sharon confirmed that Chad had agreed "several times that we would see how *1044it went; you know, we could see how it goes." And the superior court recognized that "the parties were going to see how it went and then they'll reconsider what they're going to do," which impacted its decision to grant a modification for this school year only and to leave future arrangements open for discussion. Because temporary, experimental arrangements do not constitute substantial changes in circumstances sufficient to warrant modifications of custody,15 relying on the parties' informal, temporary agreement to establish a substantial change in cireum-stances was error.
Chad argues alternatively that "the actions taken by the parties and not the mere informal agreement were the basis of the substantial change in cireumstances." And although its oral decision focused on the informal agreement, the superior court included other elements in its order denying reconsideration of its decision to modify custody:
There had been a change in cireumstances. Alexis had been integrated into her new family. She had been advised of the parents' joint decision that she should remain and go to school in Illinois. Her mother had sent her personal items and clothing to support and reflect that new arrangement. The parties set this train in motion, and to preserve her stability Alexis will reside with Chad and go to school in Illinois.
But none of these elements constitutes a substantial change in cireumstances. First, Alexis was scheduled to spend the summer with Chad in Illinois under the existing agreement. Her integration into Chad's household was a natural consequence of visitation, not a substantial change in cireum-stances, and her return to Alaska at the end of the summer was anticipated by the custody arrangement, and did not amount to a disruption in her stability. Similarly, Alexis had been in school for only two days before her mother requested her return and only one week by the time of the hearing. The superior court found in its oral decision that in light of this short duration, and in light of her young age, her starting the first grade in Illinois was not a compelling consideration. The superior court stated:
The beginning of school is not an overwhelming consideration. This is first grade.... I mean, it wouldn't-I know that school is important ... and being there with everybody else at the start might be important, but the first grade, I don't find that ... even a week or two weeks or to start late, one or the other is compelling, but that is not-just because she started a week ago isn't going to be dispositive.
Next, the superior court cited the school clothing Sharon sent as evidence that the parties had an informal agreement, but as discussed above, an informal, experimental agreement with a duration of a few weeks is insufficient to warrant a custody modification under our case law.
Nor does Alexis's expectation that she would remain in Illinois warrant a modification of custody. To rule otherwise would allow non-custodial parents to manufacture a substantial change in circumstances merely by telling a child that she will remain with the non-custodial parent. Notably, the statute does not include consideration of the child's expectations in its best-interests analysis16 It does require consideration of the child's preferences,17 but the record does not indicate that Alexis expressed a preference to stay in Ilinois rather than return to Alaska. And even had she expressed such a preference, its persuasiveness would be limited in light of her age.18 As one commentator *1045has remarked, involving young children in the process of deciding child custody matters is generally not appropriate because "they have neither the emotional and cognitive maturity, nor the capacity for moral reasoning, that is essential to participation in meaningful dialogue regarding their perspectives on divorcee outcomes and parent behaviors."19
The superior court's finding that Chad had proven the requisite substantial change in cireumstances to support a modification of custody was an abuse of discretion. Because we reverse the modification of custody for this reason, we need not reach Sharon's other arguments.20
v. CONCLUSION
Because no substantial change in cireum-stances was demonstrated, we REVERSE the superior court's decision and VACATE the order modifying custody. Alexis shall spend summer 2008 with Sharon. The parties shall then revert to the original custody arrangement under which Sharon exercises school-year custody and Chad is permitted visitation as outlined in the parenting plan.
CARPENETI, Justice, dissenting, with whom WINFREE, Justice, joins in Part IV(C).. Barrett v. Alguire, 35 P.3d 1, 5 (Alaska 2001).
. Chesser-Witmer v. Chesser, 117 P.3d 711, 715 (Alaska 2005).
. Barreit, 35 P.3d at 5.
. Morino v. Swayman, 970 P.2d 426, 428 (Alaska 1999).
. S.N.E. v. R.L.B., 699 P.2d 875, 878 (Alaska 1985).
. Nichols v. Nichols, 516 P.2d 732, 735 (Alaska 1973).
. Under the settlement agreement, Alexis was to return to Alaska three weeks before school began.
. 970 P.2d at 429.
. Id. at 427, 429-30.
. Id. at 429.
. Id.
. Id. at 433 (Fabe, J., dissenting).
. See supra note 7.
. 970 P.2d at 428.
. 970 P.2d at 429; see also id. at 430-31 (Fabe, J., dissenting).
. See AS 25.24.150(c); AS 25.20.110.
. AS 25.24.150(c)(3); AS 25.20.110. The superior court did not address Alexis's preference in its oral decision or its order on reconsideration. The superior court's silence on this factor suggests that it did not consider Alexis to be "of sufficient age and capacity to form a preference." AS 25.24.150(c)(3).
. See Veazey v. Veazey, 560 P.2d 382, 386 (Alaska 1977), superseded by statute on other grounds, ch. 63, § 30, SLA 1977, as recognized in Deivert v. Oseira, 628 P.2d 575, 579 (Alaska 1981) (suggesting that a young child's preferences are often unreliable as she can be easily influenced by the behavior of her parents, but that a rlatively mature teenager's reasoned preference carries more weight).
. Joan B. Kelly, Psychological and Legal Interventions for Parents and Children in Custody and Access Disputes: Current Research and Practice, 10 Va. J. Soc Poly & L. 129, 151 (2002).
. We do note, however, that the superior court appears to have had little information regarding Alexis's best interests beyond the testimony of Sharon and Chad. The superior court may have felt that it had sufficient information to grant a temporary, one-year modification of custody, but as we have explained, such modifications should be granted only rarely. Chesser v. Chesser-Witmer, 178 P.3d 1154, 1157 n. 10 (Alaska 2008) ("Generally, custody orders are considered final and permanent.... While a temporary order may have been appropriate under the unusual circumstances of this case, such instances should be rare.").