OPINION

delivered the opinion of the court,

We granted this appeal to determine: (1) whether parties may enter into a private agreement regarding the payment of child support outside the Child Support Guidelines; (2) whether the evidence preponderates against an award of retroactive child support in excess of the amount agreed upon by the parties; and (3) whether the plaintiff rebutted the presumption that a two-year average of income should be used to determine the amount of child support due under the guidelines. After careful consideration, we hold that a private agreement as to child support payments violates public policy, that the trial court failed to properly apply *189the Child Support Guidelines to determine the amount of child support, and that the plaintiff successfully rebutted the presumption that a two-year average of income should be used to determine the proper amount of child support. We remand the case for an application of the Child Support Guidelines to determine the amount of child support that would be owed under the guidelines and, if appropriate, for findings of fact justifying a conclusion that the application of the guidelines would be unjust or inappropriate.

Betty Berryhill was a patient of Memphis psychiatrist Dr. Charles T. Rhodes in 1975 or 1976. The parties began a sexual relationship that resulted in the birth of a child, Anika L. Berryhill, on September 5, 1977. Dr. Rhodes paid the delivery-related charges not covered by insurance. Dr. Rhodes then began paying Ms. Berryhill $200 per month. Dr. Rhodes increased the payment to $300 per month when the child was approximately six months of age. He continued to make monthly payments until Anika’s eighteenth birthday in September 1995. During Anika’s minority, Ms. Berryhill requested additional increases in support. Dr. Rhodes, however, refused these requests. In October 1995, Ms. Berryhill filed a petition to establish paternity. In addition, she requested child support from the date of the child’s birth through the period of her minority.

At the time of trial, Ms. Berryhill was forty-six years old and had been primarily employed with the State of Tennessee Division of Rehabilitation Services since 1970. From 1985 through the time of trial, she was also employed part-time at Federal Express. Ms. Berryhill testified that she maintained health insurance coverage for Anika through both jobs to ensure that her child had adequate insurance coverage. She also incurred medical and dental expenses not covered by insurance.

These amounts were not specified. Dr. Rhodes provided no insurance. He testified that he was never asked to make any contributions to the child’s medical care.

Dr. Rhodes did not visit the child. After the child’s birth, Ms. Berryhill sent a photograph of the child to Dr. Rhodes and he “became explosive.” Dr. Rhodes acknowledged that he asked Ms. Berryhill not to send any more photographs of the child. Dr. Rhodes saw his daughter in person for the first time when blood tests were conducted in December 1995.

Dr. Rhodes graduated from medical school in 1963. The record reflects that Dr. Rhodes’ income varied greatly in the years for which financial records were available. In 1988, 1989, and 1990, his income was substantially higher. In those years he reported earning $125,000 annually under a hospital contract in addition to earnings from his private practice. The hospital contract was not renewed. Since 1990, and through the time of trial, Dr. Rhodes maintained a private practice and experienced declining income. In 1990, Dr. Rhodes divorced his wife of twenty-four years. As a result of the divorce, he paid child support for two children during a portion of Anika’s minority. At the time of trial, he testified he was working part-time and contemplating retirement.

The juvenile court referee found that Dr. Rhodes was the natural father of Ani-ka L. Berryhill.1 The referee also found that the parties voluntarily entered into an agreement for Dr. Rhodes to pay the expenses incidental to the child’s birth and to pay a monthly amount of $2502 and then $300. The payments were made until the child’s majority. The referee held that the amount of support agreed to by the parties was “just and reasonable” and it would be “unfair and unreasonable to unjustly enrich the petitioner” by ordering additional support after Anika attained majority.

*190Accordingly, the juvenile court held that “the defendant’s payment of support as agreed to by the parties has satisfied his obligations under the law.” The trial court awarded attorney’s fees to Ms. Berryhill in the amount of $500. The juvenile court judge confirmed the referee’s findings.

The Court of Appeals held that the juvenile court failed to comply with the Child Support Guidelines and remanded the case to the juvenile court. Upon remand, the trial court was directed to state the amount that would have been required under the guidelines and to include a justification for deviating from the guidelines.3 In making that determination, the trial court must specifically state that the application of the guidelines would be “unjust or inappropriate” instead of “unfair and unreasonable.” Although stating that the discussion was not necessary for its decision, the Court of Appeals held that the evidence did not preponderate against the trial court’s finding that there was an implied agreement between the parties.

ANALYSIS

Ms. Berryhill contends that the lower courts erred in finding either an express or an implied agreement between her and Dr. Rhodes as to the amount of support to be paid. She argues that it was error for the juvenile court to deviate from the Child Support Guidelines and to refuse to award additional support. She also argues that the additional support should be awarded based upon a ten-year average of Dr. Rhodes’ income rather than a presumptive two-year average provided under the guidelines. Dr. Rhodes argues that retroactive awards are discretionary with the juvenile court. He also argues that Ms. Berryhill should be estopped from seeking additional support because she accepted his payments under an express or an implied agreement. We review the record of the trial court de novo with the presumption that the decision of the trial court with respect to the facts is correct unless the evidence preponderates against such factual determinations. Farrar v. Farrar, 553 S.W.2d 741, 743 (Tenn. 1977).

Private Agreement for Child Support

Ms. Berryhill contends that both the Court of Appeals and the juvenile court erred in finding that the parent of a child to whom child support is owed may enter into a private agreement4 to accept child support less than that required to be paid under the Child Support Guidelines and Tenn.Code Ann. § 36-5-101 (1995). The Court of Appeals examined the actions of the parties and determined that an “implied agreement” existed between the parties. In part, the court relied upon the language of Tenn.Code Ann. § 36 — 5—101(h) to hold that informal agreements “between adult parties should be a consideration of the court.”

Our paternity and child support statutes and the Child Support Guidelines evince a policy that children should be supported by their fathers. The paternity statutes provide a process by which the putative father can be identified. Once identified, the father is required to furnish support and education for the child.5 TenmCode Ann. § 36-2-102 (repealed in 1997; corresponding section now found at Tenn.Code Ann. § 36-2-311); Cline v. Drew, 735 S.W.2d 232, 235 (Tenn.Ct.App. 1987); Frazier v. McFerren, 55 Tenn.App. 431, 438, 402 S.W.2d 467, 471 (1964). The paternity statutes incorporate both the child support provisions pertaining to divorce decrees as *191well as the Child Support Guidelines. See Tenn.Code Ann. § 36-2-108(d) (repealed in 1997; corresponding section now found at Tenn.Code Ann. § 36-2-311), incorporating Tenn.Code Ann. § 36-5-101. The legal duty of support exists in all cases. Smith v. Puett, 506 F.Supp. 134, 142 (M.D.Tenn. 1980).

Tennessee Code Annotated § 36-5-101(h), incorporated in the paternity statute, states that any agreement reached by parents regarding child support may be affirmed, ratified, and incorporated into a divorce decree.6 This subsection contemplates that the agreement: 1) will be in writing; 2) will be approved by a court; 3) will be incorporated into a court order;7 and 4) will contain the parties’ acknowledgment that they may not alter the agreement without court approval. If the parties meet these requirements, they may enter into a valid agreement to set child support. In this case, the parties met none of the requirements of the statute. We fail to see how the enactment of § 36-5-101(h) evinces a legislative intent to uphold private agreements that fail to comply with the statute in any respect.

Although this Court has not specifically addressed the issue of the validity of a private agreement for payment of child support, other states that have considered the issue have found such agreements violate public policy. In Paul M. v. Teresa M., 36 Ark.App. 116, 818 S.W.2d 594 (1991), the Arkansas appellate court rejected a father’s argument in a paternity action that he was relieved of his support obligation because the child’s mother had agreed to take full responsibility for the child. “Insofar as the agreement at issue here represents an attempt to permanently deprive the child of support, it is void as against public policy.” Id. at 596. The court stated that a duty of support cannot be bargained away permanently to the detriment of the child. Similarly, the court has held that an agreement not to seek increases in child support is void as against public policy. Id. at 595. “These holdings are based on principles that the interests of minors have always been the subject of jealous and watchful care by the courts....” Id . Likewise, in Worthington v. Worthington, 250 Ga. 730, 301 S.E.2d 44 (1983), the Georgia Supreme Court held that a lump sum payment of $10,000 to the child’s mother did not release the father from his child support obligations. “Children, legitimate or illegitimate, are not property, and absent a clear legislative declaration otherwise their support rights may not be bartered away by their parents.” Id. at 46.

Courts in several jurisdictions have found that a child’s right to support cannot be bargained away by a parent to the child’s detriment. Davis v. Office of Child Support Enforcement, 322 Ark. 352, 908 S.W.2d 649, 651 (1995); see also Gammon v. Cobb, 335 So.2d 261, 266-67 (Fla. 1976) (noting that the mother is the trustee who receives funds and simply converts them into relief for the children); Livsey v. *192Livsey, 229 Ga. 368, 191 S.E.2d 859, 860 (1972); Tuer v. Niedoliwka, 92 Mich.App. 694, 285 N.W.2d 424, 426 (1979) (adopting holding that “an illegitimate child’s right to support from a putative father cannot be contracted away by its mother, and that any release or compromise executed by the mother is invalid to the extent that it purports to affect the rights of the child”); State v. Dongher, 47 Minn. 436, 50 N.W. 475, 475 (1891) (rejecting father’s argument that he was released from child support obligations by the payment of $100 to the mother); Fox v. Hohenshelt, 19 Or. App. 617, 528 P.2d 1376, 1381 (1974) (holding that two parties should not be able to prejudice the rights of a third and that a contract between mother and putative father of illegitimate child cannot preclude future filiation proceedings for purposes of child support without judicial scrutiny and approval); Diehl v. Mulhem, 406 Pa.Super. 422, 594 A.2d 692, 695 (1991); Shelby J.S. v. George L.H., 181 W.Va. 154, 381 S.E.2d 269, 271 (1989) (noting court’s caution in permitting a mother to settle child support claims with natural father).

We find the holdings and reasoning of these courts to be persuasive. Tennessee’s statutory provisions for the establishment of paternity and support and the Child Support Guidelines evince a policy that fathers will support their children. Private agreements used to circumvent the obligations set forth in the statutes and guidelines contravene that policy.

Additional Amount of Support

Although we have determined that any private agreement, whether express or implied, would violate public policy, we recognize that Dr. Rhodes made monthly payments of $200 and $300 during Anika’s minority. We must, therefore, address whether the evidence preponderates against the trial court’s conclusion that the amounts paid were “just and reasonable” and that it would be “unfair and unreasonable to unjustly enrich” Ms. Berryhill by ordering additional amounts of support subsequent to Anika’s majority. For the reasons below, we conclude that a remand to the juvenile court is necessary.

The legislature has provided for retroactive awards by statute and by the incorporation of the Child Support Guidelines promulgated by the Tennessee Department of Human Services, Child Support Services Division. Retroactive child support is available whether the child is a minor or whether the child has reached the age of majority and brings the claim within time permitted by the statute. Tenn.Code Ann. § 36-2-103(b)(l) (repealed in 1997; corresponding section at Tenn.Code Ann. § 36-2-306). Furthermore, courts are required to apply the Child Support Guidelines as a rebuttable presumption in determining support, and the 1994 guidelines explicitly provide “that the rebuttable presumption must be applied to all child support awards even if the order is being sought for a retroactive period before October 18, 1989.” Tenn. Comp.R. & Regs. ch. 1240-2-4-.01(2) (emphasis added). This Court has held that the guidelines “carry what amounts to a legislative mandate.” Nash v. Mulle, 846 S.W.2d 803, 804 (Tenn. 1993). Accordingly, the mere action of seeking an award of retroactive child support within the time frame permitted by statute cannot render a request for child support either “unjust” or “inappropriate.”

The Court of Appeals in this case recognized the guidelines apply as a rebuttable presumption regarding the amount of child support to be paid. The court, however, stated that the presumption is not to be construed as an abrogation of this Court’s statement in Coleman v. Clay, 805 S.W.2d 752, 755 (Tenn. 1991), that a court “has broad discretion to determine the amount of such a retroactive award.” This Court acknowledged in Coleman that a father is responsible for the support of his child and that this obligation arises at the date of the child’s birth. Id. at 754-55. This Court further stated that a juvenile court judge has broad discretion to determine *193the amount of such a retroactive award. Id. at 755.

The Child Support Guidelines, however, were silent as to retroactive awards when this Court decided Coleman. Subsequent to the decision in Coleman, retroactivity provisions were added to the Child Support Guidelines. The specific inclusion of these provisions in 1994 limited the courts’ discretion in setting amounts of child support. While the juvenile court continues to have discretion in making awards of child support, that discretion must be -exercised within the strictures of the Child Support Guidelines.

The Court of Appeals remanded the case to the trial court to require the trial court to state the amount of child support that would have been required under the guidelines as well as a justification for deviation from the guidelines. We agree that a remand is required in this case for the trial court to make appropriate findings of fact. The guidelines provide a general formula for calculating the appropriate amount of child support based on an obligor’s income and include a procedure which permits limited deviation downward from the general formula. Tenn.Comp.R. & Regs. ch. 1240-2-4-.04(2)(a) & (b) and (4) (1994). The guidelines also mandate a deviation upward under certain circumstances. Tenn.Comp.R. & Regs. ch. 1240-2-4-.04(l)(a)-(d) and (f) (1994). The guidelines “are a minimum base for determining child support obligations.” Tenn. Comp.R. & Regs. ch. 1240-2-4-02(5).

“[T]he guidelines expressly provide for downward deviation where the obligee has utterly ceased to care for the child(ren); where the obligee clearly has a lower level of child care expense than that assumed in the guidelines; and where the obligor is saddled with ‘extreme economic hardship.’ ” Jones v. Jones, 930 S.W.2d 541, 545 (Tenn. 1996); Tenn.Comp.R. & Regs, ch. 1240-2-4-.04(2)(a)-(b), -.04(4). “Although the rule does not purport to set forth an exhaustive list of instances in which downward deviation is allowed, these specific instances nevertheless are a powerful indication as to the types of situations in which it is contemplated under the guidelines.” Jones, 930 S.W.2d at 545. The guidelines indicate that the court “shall” increase the award if the obligor is not providing health insurance, if the obli-gor is exercising less than average visitation, if extraordinary medical and educational expenses exist, or if the court finds that equity requires it. Tenn. Comp.R. & Regs. ch. 1240-2-4-04(1).

After determining the amount of child support under the guidelines, the trial court may then consider whether the evidence is sufficient to rebut the presumption. To deviate from the presumptive amount, a court must enter:

a written or specific finding on the record that the application of the guidelines would be unjust or inappropriate in a particular case ... in order to provide for the best interest of the child or the equity between the parties and the court must show what the child support award would have been without the deviation.

Tenn.Comp.R. & Regs. ch. 1240-2-4-,01(2)(3), ch. 1240-2-4-02(7)(1994); see also Tenn.Code Ann. § 36-5-101(e)(l).

Time period for computation of retroactive child support

On remand the trial court will determine the amount of child support that would be due under the guidelines as well as the appropriateness of any deviation. In making the initial determination of child support, the trial court must determine the gross income of Dr. Rhodes during the time periods in question. In setting retroactive awards, the guidelines provide that the obligor’s income for the last two years is presumed to be correct unless rebutted by either party. Tenn.Comp.R. & Regs, ch. 1240-2-4-.04(l)(e).8 We must there*194fore consider whether Ms. Berryhill has successfully rebutted this presumption. We conclude that on the record before us Ms. Berryhill has done so.

Dr. Rhodes produced tax returns for ten years, 1983 and 1987-1995. Using those returns, an accountant testified that Dr.

Rhodes’ gross income was as follows:

1995 $ 51,250 1990 $333,856

1994 $ 47,490 1989 $294,067

1993 $ 80,286 1988 $235,553

1992 $123,057 1987 $ 98,248

1991 $173,427 1983 $ 30,388

Doctor Rhodes testified that his income reported in 1988, 1989, and 1990 was unusually high because he entered into a contract with a hospital and earned $125,000 per year in addition to his private practice. Dr. Rhodes testified that at the time of trial in 1996 his practice had become close to part-time and that he was contemplating retirement.

Dr. Rhodes graduated from medical school in 1963. Using the last two years of his income would unfairly emphasize a time when it appears Dr. Rhodes was winding down his medical practice. Furthermore, the obligation or duty of support spanned eighteen years-the entire life of the child. We conclude that Ms. Berryhill successfully rebutted the presumption that a two-year average should apply. She demonstrated that a representation of income over a longer period would be more appropriate. Based upon this record, however, it is unclear that the ten-year average proposed by Ms. Berryhill would be appropriate.

Ms. Berryhill presented testimony from an accountant who made certain calculations based upon Dr. Rhodes’ income over ten years. The accountant acknowledged that the information he used was not complete and may not have fully taken into account Dr. Rhodes’ other child support obligations when determining his average income. Although Dr. Rhodes’ counsel had requested additional income tax returns from the Internal Revenue Service, those records were not available at the time of trial. Counsel for Dr. Rhodes elected to go forward with the available records.

CONCLUSION

We conclude that private agreements for the payment of child support violate public policy. We remand the case for an application of the Child Support Guidelines to determine the amount of child support that would be owed under the guidelines and, if appropriate, for findings of fact justifying a conclusion that the application of the guidelines would be unjust or inappropriate.

Upon remand, Ms. Berryhill should be permitted to offer additional evidence as to her medical and dental expense claims. We hold that Ms. Berryhill successfully rebutted the presumption that a two-year average of Dr. Rhodes’ income should be used in determining the amount of child support due under the guidelines. Dr. Rhodes, however, should be permitted to offer additional evidence regarding his income for the years in question. The juvenile court will then determine the appropriate time period for averaging Dr. Rhodes’ income. Dr. Rhodes shall receive credit for the monthly payments he previously made. The case is remanded to the juvenile court for further proceedings consistent with this opinion. Costs of this appeal are to be taxed against Dr. Rhodes.

BIRCH, J., filed a dissenting and concurring opinion, in which BARKER, J., joined.

with whom Justice BARKER joins, concurring and dissenting.

I fully concur in the majority’s decision to remand this case to determine whether deviation from the guidelines is appropriate. I cannot, however, accept the majority’s analysis of private agreements for child support. To the extent that the majority opinion suggests that such agreements are per se void, and thus never enforceable, I must respectfully dissent.1

It is my opinion that parents of non-marital children should be permitted, indeed encouraged, to agree upon child support. The parties must, however, fully realize that any such private agreement is always subject to modification by the appropriate judicial authority. Viewed in this manner, one can reasonably predict that private agreements which meet applicable criteria will most likely be approved when subjected to court scrutiny. Conversely, if the agreement fails in this regard, then it is unlikely to receive court approval.

Private agreements for child support have been statutorily encouraged. As the majority notes, TenmCode Ann. § 36-5-101(h) is the statutory provision governing private agreements for child support. Tennessee Code Annotated § 36-5-101(h) provides that:

[njothing in this section shall be construed to prevent affirmation, ratification and incorporation in a decree of an agreement between the parties as to child support and maintenance of a party or as to child support.

Tenn.Code Ann. § 36-5-101(h) (Supp. 1998) (emphasis added). In interpreting this statute, I am reminded that a “basic rule of statutory construction is to ascertain and give effect to the intention and purpose of the legislature.” Carson Creek Vacation Resorts, Inc. v. State Dep’t. of Revenue, 865 S.W.2d 1, 2 (Tenn. 1993). In so doing, a court must not “unduly restrict! ] or expand! ] a statute’s coverage beyond its intended scope.” Worley v. Weigels Inc., 919 S.W.2d 589, 593 (Tenn. 1996) (quoting Owens v. State, 908 S.W.2d 923, 926 (Tenn. 1995)). Rather, a statute’s purpose must be ascertained from the plain and ordinary meaning of its language. Westland West Community Ass’n v. Knox County, 948 S.W.2d 281, 283 (Tenn. 1997). Finally, a court must not question the “reasonableness of [a] statute or substitute] its own policy judgments] for those of the legislature .” BellSouth Telecomms., Inc. v. Greer, 972 S.W.2d 663, 673 (Tenn.Ct.App. 1997). Thus, a court must “presume that the legislature says in a statute what it means and means in a statute what it says there.” Id. A statute, therefore, must be construed as it is written. See Jackson v. Jackson, 186 Tenn. 337, 342, 210 S.W.2d 332, 334 (1948).

Applying these rules of statutory construction to Tenn.Code Ann. § 36-5-101(h), I find the statute clear and unambiguous. Tennessee Code Annotated § 36-5-101(h) authorizes individuals to enter into private agreements for child support and places the enforcement of such agreements in the discretion of the trial court. When “the language contained within the four corners of a statute is plain, clear and unambiguous, the duty of the courts is simple and obvious, ‘to say sic lex scripta, and obey it.’ ” Hawks v. City of Westmoreland, 960 S.W.2d 10,16 (Tenn. 1997) (quoting Miller v. Childress, 21 Tenn. (2 Hum.) 320, 321-22 (1841)). Accordingly, in cases such as this, courts are not at liberty to depart from the words of *196the statute. Id. Thus, under Tenn.Code Ann. § 36-5-101(h), parties are permitted to enter into private agreements for child support, the enforceability of which remains within the purview of the court. See Tenn.Code Ann. § 36-5-101(h) (1996). An interpretation to the contrary would unduly restrict the statute’s intended scope. See Worley, 919 S.W.2d at 593.

Despite our statute’s plain language, the majority contends that private agreements for child support contravene public policy.2 “[T]he determination of public policy is primarily a function of the legislature,” and the judiciary determines “public policy in the absence of any constitutional or statutory declaration.” Alcazar v. Hayes, 982 S.W.2d 845, 851 (Tenn. 1998). In promulgating Tenn.Code Ann. § 36-5-101(h), our General Assembly has made clear that private agreements for child support are consistent with the public policy and ensured that such agreements do not circumvent the child support guidelines or our statutory provisions relating to child support. See TenmCode Ann. § 36-5-101(h) (Supp. 1998) (“In any such agreement, the parties must affirmatively acknowledge that no action by the parties will be effective to reduce child support after the due date of each payment, and that they understand that court approval must be obtained before child support can be reduced, unless such payments are automatically reduced or terminated under the terms of the agreement.”). Even if we should disagree with such a policy, it is not for this Court to question the statute’s reasonableness or substitute our own policy judgments for those of the General Assembly. BellSouth Telecomms., Inc., 972 S.W.2d at 673.

Additionally, under Tenn.Code Ann. § 36-5-101(h), a court can refuse to enforce a private agreement for child support. This discretion serves as a check against agreements which circumvent the obligations set forth in the guidelines or our statutes.

Indeed, as stated earlier, parents of non-marital children should be encouraged to enter into private agreements to support their children. I am perturbed by the result created by the majority opinion. Here, a father who never sought to evade parental responsibility as to paternity or support agreed with the child’s mother to pay a sum certain every month for the support of the child. This father faithfully and dutifully made these payments up to the child’s eighteenth birthday. What more can society rightfully require of him?